Queryable::First<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)
Returns the first element of a sequence that satisfies a specified condition.
Assembly: System.Core (in System.Core.dll)
[ExtensionAttribute] public: generic<typename TSource> static TSource First( IQueryable<TSource>^ source, Expression<Func<TSource, bool>^>^ predicate )
Type Parameters
- TSource
The type of the elements of source.
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: TSourceThe first element in source that passes the test in predicate.
Usage Note
In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is nullptr. |
| InvalidOperationException | No element satisfies the condition in predicate. -or- The source sequence is empty. |
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 First<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling First<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 First<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 specified by predicate.
Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003
The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.