Queryable.FirstOrDefault<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)
Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.
Assembly: System.Core (in System.Core.dll)
public static TSource FirstOrDefault<TSource>( this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate )
Parameters
- source
-
Type:
System.Linq.IQueryable<TSource>
An IQueryable<T> to return an element from.
- predicate
-
Type:
System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test each element for a condition.
Return Value
Type: TSourcedefault(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.
Type Parameters
- TSource
The type of the elements of source.
| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is null. |
This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.
The FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.
The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source that satisfies the condition in predicate, or a default value if no element satisfies the condition.
The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) by passing in a predicate. In the second query, there is no element in the sequence that satisfies the condition.
string[] names = { "Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu" }; // Get the first string in the array that is longer // than 20 characters, or the default value for type // string (null) if none exists. string firstLongName = names.AsQueryable().FirstOrDefault(name => name.Length > 20); Console.WriteLine("The first long name is '{0}'.", firstLongName); // Get the first string in the array that is longer // than 30 characters, or the default value for type // string (null) if none exists. string firstVeryLongName = names.AsQueryable().FirstOrDefault(name => name.Length > 30); Console.WriteLine( "There is {0} name that is longer than 30 characters.", string.IsNullOrEmpty(firstVeryLongName) ? "NOT a" : "a"); /* This code produces the following output: The first long name is 'Andersen, Henriette Thaulow'. There is NOT a name that is longer than 30 characters. */
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1