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: generic<typename TSource> [ExtensionAttribute] static TSource FirstOrDefault( 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.
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