CSharp Driver LINQ Tutorial

Introduction¶

This tutorial covers the support for LINQ queries added in the 1.4 release of the C# driver.

You should already have read at least the CSharp Driver Tutorial introduction to the C# driver.

Quickstart¶

First, add the following additional using statement to your program:

using MongoDB.Driver.Linq;

Then, get a reference to a collection variable in the usual way:

var collection = database.GetCollection<TDocument>("collectionname");

The basic idea behind writing a LINQ query is to start from a collection variable and begin the LINQ query by calling the AsQueryable<TDocument>() method. After that it’s all standard LINQ.

For example:

var query =
    from e in collection.AsQueryable<Employee>()
    where e.FirstName == "John"
    select e;

foreach (var employee in query)
{
    // process employees named "John"
}

You can also write queries using lambda syntax. The previous query would be written using lambda syntax like this:

var query =
    collection.AsQueryable<Employee>()
    .Where(e => e.FirstName == "John")
    .Select(e => e); // this trivial projection is optional when using lambda syntax

The C# compiler translates all queries written using query syntax into lambda syntax internally anyway, so there is no performance advantage or penalty to choosing either style. You can also mix and match the styles, which can be useful when using query operators that are not supported by the query syntax.

All the code samples in this tutorial show both the query syntax and the lambda syntax for each query operator and supported where clauses.

Only LINQ queries that can be translated to an equivalent MongoDB query are supported. If you write a LINQ query that can’t be translated you will get a runtime exception and the error message will indicate which part of the query wasn’t supported.

Supported LINQ query operators¶

This section documents the supported LINQ query operators.

• Any

  Without a predicate Any just tests whether the collection has any documents.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Any();
  // or
  var result =
      collection.AsQueryable<C>()
      .Any();
  

• Any (with predicate)

  With a predicate Any tests whether the collection has any matching documents.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Any(c => c.X == 1);
  // or
  var result =
      collection.AsQueryable<C>()
      .Any(c => c.X == 1);
  

  Note that the predicate can be provided either by a where clause or as an argument to Any, so the following are equivalent to the previous query.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X == 1
      select c)
      .Any();
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X == 1)
      .Any();
  

  Any with a predicate is not supported after a projection (at least not yet). So the following is not valid:

  var result =
      collection.AsQueryable<C>()
      .Select(c => c.X)
      .Any(x => x == 1);
  

  You can usually rewrite such a query by putting an equivalent where clause before the projection (in which case you can drop the projection).

• Count

  Without a predicate Count just returns the number of documents in the collection.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Count();
  // or
  var result =
      collection.AsQueryable<C>()
      .Count();
  

• Count (with predicate)

  With a predicate Count returns the number of documents that match the predicate.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Count(c => c.X == 1);
  // or
  var result =
      collection.AsQueryable<C>()
      .Count(c => c.X == 1);
  

  Note that the predicate can be provided either by a where clause or as an argument to Count, so the following are equivalent to the previous query.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X == 1
      select c)
      .Count();
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X == 1)
      .Count();
  

  Count with a predicate is not supported after a projection (at least not yet). So the following is not valid:

  var result =
      collection.AsQueryable<C>()
      .Select(c => c.X)
      .Count(x => x == 1);
  

  You can usually rewrite such a query by putting an equivalent where clause before the projection (in which case you can drop the projection).

• Distinct

  Distinct returns the unique values of a field or property of the documents in the collection. You use a projection to identify the field or property whose distinct values you want.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.X)
      .Distinct();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.X)
      .Distinct();
  

  The projection must select a particular field or property of the document. If the value of that field or property is represented in MongoDB as an array you can also use array indexing to select an item from the array.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.A[i])
      .Distinct();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.A[i])
      .Distinct();
  

• ElementAt

  ElementAt returns a particular document from a result set. Often you will combine this with a sort order.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X > 0
      orderby c.X
      select c)
      .ElementAt(index);
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0)
      .OrderBy(c => c.X)
      .ElementAt(index);
  

  If the result set has fewer documents than index ElementAt throws an exception.

• ElementAtOrDefault

  ElementAtOrDefault is just like ElementAt except that if there are fewer documents than index it returns null instead of throwing an exception.

• First

  First returns the first document from a result set. Often you will combine this with a sort order.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X > 0
      orderby c.X
      select c)
      .First();
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0)
      .OrderBy(c => c.X)
      .First();
  

  If the result set has no documents First throws an exception.

• First (with predicate)

  This overload of First allows you to provide a predicate as an argument to First. This is an alternative to using a where clause.

  var result =
      (from c in collection.AsQueryable<C>()
      orderby c.X
      select c)
      .First(c => c.X > 0);
  // or
  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .First(c => c.X > 0);
  

  First with a predicate is not supported after a projection (at least not yet). So the following is not valid:

  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Select(c => c.X)
      .Count(x => x > 0);
  

  You can usually rewrite such a query by putting an equivalent where clause before the projection.

  If the result set has no documents First with a predicate throws an exception.

• FirstOrDefault

  FirstOrDefault is just like First except that if there are no matching documents it returns null instead of throwing an exception.

• FirstOrDefault (with predicate)

  FirstOrDefault with a predicate is just like First with a predicate except that if there are no matching documents it returns null instead of throwing an exception.

• Last

  Last returns the last document from a result set. Often you will combine this with a sort order.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X > 0
      orderby c.X
      select c)
      .Last();
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0)
      .OrderBy(c => c.X)
      .Last();
  

  If the result set has no documents Last throws an exception.

• Last (with predicate)

  This overload of Last allows you to provide a predicate as an argument to Last. This is an alternative to using a where clause.

  var result =
      (from c in collection.AsQueryable<C>()
      orderby c.X
      select c)
      .Last(c => c.X > 0);
  // or
  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Last(c => c.X > 0);
  

  Last with a predicate is not supported after a projection (at least not yet). So the following is not valid:

  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Select(c => c.X)
      .Last(x => x > 0);
  

  You can usually rewrite such a query by putting an equivalent where clause before the projection.

  If the result set has no documents Last throws an exception.

• LastOrDefault

  LastOrDefault is just like Last except that if there are no matching documents it returns null instead of throwing an exception.

• LastOrDefault (with predicate)

  LastOrDefault with a predicate is just like Last with a predicate except that if there are no matching documents it returns null instead of throwing an exception.

• LongCount

  LongCount is just like Count except that the return value is a 64-bit integer instead of a 32-bit integer.

• LongCount (with predicate)

  LongCount with a predicate is just like Count with a predicate except that the return value is a 64-bit integer instead of a 32-bit integer.

• Max

  Max returns the maximum value of a field or property of the documents in the collection. You use a projection to identify the field or property whose maximum value you want.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.X)
      .Max();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.X)
      .Max();
  

  The projection must select a particular field or property of the document. If the value of that field or property is represented in MongoDB as an array you can also use array indexing to select an item from the array.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.A[i])
      .Max();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.A[i])
      .Max();
  

• Max (with selector)

  This overload of Max lets you select the field or property whose maximum value you want as an argument to Max instead of to Select.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Max(c => c.X);
  // or
  var result =
      collection.AsQueryable<C>()
      .Max(c => c.X);
  

• Min

  Min returns the minimum value of a field or property of the documents in the collection. You use a projection to identify the field or property whose minimum value you want.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.X)
      .Min();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.X)
      .Min();
  

  The projection must select a particular field or property of the document. If the value of that field or property is represented in MongoDB as an array you can also use array indexing to select an item from the array.

  var result =
      (from c in collection.AsQueryable<C>()
      select c.A[i])
      .Min();
  // or
  var result =
      collection.AsQueryable<C>()
      .Select(c => c.A[i])
      .Min();
  

• Min (with selector)

  This overload of Min lets you select the field or property whose minimum value you want as an argument to Min instead of to Select.

  var result =
      (from c in collection.AsQueryable<C>()
      select c)
      .Min(c => c.X);
  // or
  var result =
      collection.AsQueryable<C>()
      .Min(c => c.X);
  

• OrderBy

  OrderBy is used to specify an ascending sort order for the result set.

  var query =
      from c in collection.AsQueryable<C>()
      orderby c.X
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X);
  

• OrderByDescending

  OrderByDescending is used to specify a descending sort order for the result set.

  var query =
      from c in collection.AsQueryable<C>()
      orderby c.X descending
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderByDescending(c => c.X);
  

• Select

  Select is used to project a new result type from the matching documents. In the 1.4 version of the C# driver a projection must typically be the last operation (with a few exceptions like Distinct, Max and Min).

  var query =
      from c in collection.AsQueryable<C>()
      select new { c.X, c.Y };
  // or
  var query =
      collection.AsQueryable<C>()
      .Select(c => new { c.X, c.Y });
  

• Single

  Single returns the first and only document from a result set.

  var result =
      (from c in collection.AsQueryable<C>()
      where c.X > 0
      orderby c.X
      select c)
      .Single();
  // or
  var result =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0)
      .OrderBy(c => c.X)
      .Single();
  

  If the result set has no documents or multiple documents Single throws an exception.

• Single (with predicate)

  This overload of Single allows you to provide a predicate as an argument to Single . This is an alternative to using a where clause.

  var result =
      (from c in collection.AsQueryable<C>()
      orderby c.X
      select c)
      .Single(c => c.X > 0);
  // or
  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Single(c => c.X > 0);
  

  Single with a predicate is not supported after a projection (at least not yet). So the following is not valid:

  var result =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Select(c => c.X)
      .Single(x => x > 0);
  

  You can usually rewrite such a query by putting an equivalent where clause before the projection.

  If the result set has no documents or multiple documents Single throws an exception.

• SingleOrDefault

  SingleOrDefault is just like Single except that if there are no matching documents it returns null instead of throwing an exception.

• SingleOrDefault (with predicate)

  SingleOrDefault with a predicate is just like Single with a predicate except that if there are no matching documents it returns null instead of throwing an exception.

• Skip

  Use Skip to specify how many documents to skip from the beginning of the result set. Often you will combine Skip with a sort order.

  var query =
      (from c in collection.AsQueryable<C>()
      orderby c.X
      select c)
      .Skip(100);
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Skip(100);
  

• Take

  Use Take to specify how many documents to return from the server. When combining Take with Skip often you will also specify a sort order.

  var query =
      (from c in collection.AsQueryable<C>()
      orderby c.X
      select c)
      .Skip(100)
      .Take(100);
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .Skip(100)
      .Take(100);
  

• ThenBy

  ThenBy is used to specify an additional ascending sort order for the result set.

  var query =
      from c in collection.AsQueryable<C>()
      orderby c.X, c.Y
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .ThenBy(c => c.Y);
  

• ThenByDescending

  ThenByDescending is used to specify an additional descending sort order for the result set.

  var query =
      from c in collection.AsQueryable<C>()
      orderby c.X, c.Y descending
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .OrderBy(c => c.X)
      .ThenByDescending(c => c.Y);
  

• Where

  A where clause is used to specify which documents the query should return. A where clause is a C# expression that maps the query document type to a boolean value. If the expression returns true the document “matches” the query and is included in the result set.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X > 0
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0);
  

  Sometimes a predicate can be supplied in other places besides a where clause, and it is also possible to have multiple where clauses. When multiple predicates are involved they are combined into a single composite predicate by combining the individual predicates with the && operator.

  For example, the following queries are equivalent:

  var query =
      (from c in collection.AsQueryable<C>()
      where c.X > 0
      where c.Y > 0)
      .First(c.Z > 0);
  // or
  var query =
      (from c in collection.AsQueryable<C>()
      where c.X > 0 && c.Y > 0 && c.Z > 0)
      .First();
  

Supported where clauses¶

This section documents the supported where clauses.

As mentioned earlier, not all C# expressions are supported as a where clause. You can use this documentation as a guide to what is supported, or you can just try an expression and see if it works (a runtime exception is thrown if the where clause is not supported).

Where clauses are typically introduced using the Where query operator, but the same expressions are supported wherever a predicate is called for. In some cases multiple where clauses and predicates will be combined, in which case they are combined with the && operator.

• && (And operator)

  Sub-expressions can be combined with the && operator to test whether all of them are true.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X > 0 && c.Y > 0
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0 && c.Y > 0);
  

  This is translated to the following MongoDB query:

  { X : { $gt : 0 }, Y : { $gt : 0 } }
  

  In some cases the And query can’t be flattened as shown, and the $and operator will be used. The following example matches documents where X is both a multiple of 2 and a multiple of 3:

  var query =
      from c in collection.AsQueryable<C>()
      where (c.X % 2 == 0) && (c.X % 3 == 0)
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => (c.X % 2 == 0) && (c.X % 3 == 0));
  

  This is translated to the following MongoDB query using $and:

  { $and : [{ X : { $mod : [2, 0] } }, { X : { $mod : [3, 0] } }] }
  

• Any

  This method is used to test whether an array field or property contains any items.

  var query =
      from c in collection.AsQueryable<C>()
      where c.A.Any()
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.A.Any());
  

  matches any document where A has 1 or more items.

  This is translated to the following MongoDB query:

  { A : { $ne : null, $not : { $size : 0 } } }
  

• Boolean constant

  This form is mostly for completeness. You will probably use it rarely. It allows a boolean constant to be used to either match or not match the document.

  var query =
      from c in collection.AsQueryable<C>()
      where true
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => true);
  

  This is translated to the following MongoDB query:

  { _id : { $exists : true } }
  

  Which matches all documents since the _id is a mandatory field.

• Boolean field or property

  A boolean field or property of the document doesn’t have to be compared to true, it can just be mentioned in the where clause and there is an implied comparison to true.

  var query =
      from c in collection.AsQueryable<C>()
      where c.B
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.B);
  

  This is translated to the following MongoDB query:

  { B : true }
  

• Contains (Enumerable method)

  This method is used to test whether an array (or array-like) field or property contains a particular value:

  var query =
      from c in collection.AsQueryable<C>()
      where c.A.Contains(123)
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.A.Contains(123));
  

  This is translated to the following MongoDB query:

  { A : 123 }
  

  This translation relies on the way array fields are treated by the MongoDB query language.

• Contains (string method)

  This method is used to test whether a string field or property of the document contains a particular substring.

  var query =
      from c in collection.AsQueryable<C>()
      where c.S.Contains("abc")
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.S.Contains("abc"));
  

  This is translated to the following MongoDB query (using regular expressions):

  { S : /abc/ }
  

• ContainsAll (LINQ to MongoDB extension method)

  This method is used to test whether an array (or array-like) field or property contains all of the provided values.

  var query =
      from c in collection.AsQueryable<C>()
      where c.A.ContainsAll(new[] { 1, 2, 3 })
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.A.ContainsAll(new[] { 1, 2, 3 }));
  

  This is translated to the following MongoDB query:

  { A : { $all : [1, 2, 3] } }
  

• ContainsAny (LINQ to MongoDB extension method)

  This method is used to test whether an array (or array-like) field or property contains any of the provided values.

  var query =
      from c in collection.AsQueryable<C>()
      where c.A.ContainsAny(new[] { 1, 2, 3 })
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.A.ContainsAny(new[] { 1, 2, 3 }));
  

  This is translated to the following MongoDB query:

  { A : { $in : [1, 2, 3] } }
  

• Count method (array length)

  This method is used to test whether an enumerable field or property has a certain count of items.

  var query =
      from c in collection.AsQueryable<C>()
      where c.L.Count() == 3
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.L.Count() == 3);
  

  This is translated to the following MongoDB query:

  { L : { $size: 3 } }
  

• Count property (array length)

  This property is used to test whether a list (or list-like) field or property has a certain count of items.

  var query =
      from c in collection.AsQueryable<C>()
      where c.L.Count == 3
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.L.Count == 3);
  

  This is translated to the following MongoDB query:

  { L : { $size: 3 } }
  

• EndsWith (string method)

  This method is used to test whether a string field or property of the document ends with a particular substring.

  var query =
      from c in collection.AsQueryable<C>()
      where c.S.EndsWith("abc")
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.S.EndsWith("abc"));
  

  This is translated to the following MongoDB query (using regular expressions):

  { S : /abc$/ }
  

• enum comparisons (==, !=, <, <=, >, >=)

  enum fields or properties can be compared to constants of the same enum type. The relative comparison are based on the value of the underlying integer type.

  public enum E { None, A, B };
  
  var query =
      from c in collection.AsQueryable<C>()
      where c.E == E.A
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.E == E.A);
  

  This is translated to the following MongoDB query:

  { E : 1 }
  

  The LINQ implementation takes the representation of serialized values into account, so if you have configured your class map to store enums as string values instead of integer values the MongoDB query would instead be:

  { E : "A" }
  

• In (LINQ to MongoDB extension method)

  The In method is used to test whether a field or property is equal any of a set of provided values.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X.In(new [] { 1, 2, 3 })
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c.X.In(new [] { 1, 2, 3 }));
  

  This is translated to the following MongoDB query:

  { X : { $in : [1, 2, 3] } }
  

• Inject

  Inject is a pseudo-method that is used to inject a lower level MongoDB query into a LINQ query. The following query looks for X values that are larger than 0 and are 64-bit integers.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X > 0 && Query.Type("X", BsonType.Int64).Inject()
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0 && Query.Type("X", BsonType.Int64).Inject());
  

  This is translated to the following MongoDB query:

  { X : { $gt : 0, $type : 18 } }
  

• IsMatch (regular expression method)

  This method is used to test whether a string field or property matches a regular expression.

  var regex = new Regex("^abc");
  var query =
      from c in collection.AsQueryable<C>()
      where regex.IsMatch(c.S)
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => regex.IsMatch(c.S));
  

  This is translated to the following MongoDB query:

  { S : /^abc/ }
  

  You can also use the static IsMatch method.

  var query =
      from c in collection.AsQueryable<C>()
      where Regex.IsMatch(c.S, "^abc")
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => Regex.IsMatch(c.S, "^abc"));
  

  This is translated to the following MongoDB query:

  { S : /^abc/ }
  

• Length (array length)

  This method is used to test whether an array (or array-like) field or property has a certain count of items.

  var query =
      from c in collection.AsQueryable<C>()
      where c.A.Length == 3
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.A.Length == 3);
  

  This is translated to the following MongoDB query:

  { A : { $size: 3 } }
  

• % (Mod operator)

  This operator is used to test the result of the mod operator against a field or property of the document. The following query matches all the documents where X is odd.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X % 2 == 1
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X % 2 == 1);
  

  This is translated to the following MongoDB query:

  { X : { $mod : [2, 1] } }
  

• ! (Not operator)

  The ! operator is used to reverse the sense of a test.

  var query =
      from c in collection.AsQueryable<C>()
      where !(c.X > 1)
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => !(c.X > 1));
  

  This is translated into the following MongoDB query:

  { X : { $not : { $gt : 1 } } }
  

  注解

  !(c.X > 1) is not equivalent to (c.X <= 1) in cases where c.X is missing or does not have a numeric type.

• Numeric comparisons (==, !=, <, <=, >, >=)

  Numeric fields or properties can be compared using any of the above operators.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X == 0 && c.Y < 100
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0 && c.Y < 100);
  

  This is translated into the following MongoDB query:

  { X : 0, Y : { $lt : 100 } }
  

• || (Or operator)

  Sub-expressions can be combined with the || operator to test whether any of them is true.

  var query =
      from c in collection.AsQueryable<C>()
      where c.X > 0 || c.Y > 0
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.X > 0 || c.Y > 0);
  

  This is translated to the following MongoDB query:

  { $or : [{ X : { $gt : 0 } }, { Y : { $gt : 0 } }] }
  

• StartsWith (string method)

  This method is used to test whether a string field or property of the document starts with a particular substring.

  var query =
      from c in collection.AsQueryable<C>()
      where c.S.StartsWith("abc")
      select c;
  // or
  var query =
      collection.AsQueryable<C>()
      .Where(c => c.S.StartsWith("abc"));
  

  This is translated to the following MongoDB query (using regular expressions):

  { S : /^abc/ }