You can use this delegate to represent a method that can be passed as a parameter without explicitly declaring a custom delegate. The method must correspond to the method signature that is defined by this delegate. This means that the encapsulated method must have no parameters and must return a value.
.gif) Note: |
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To reference a method that has no parameters and that returns
void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the Action delegate instead.
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When you use the Func<(Of <(TResult>)>) delegate, you do not have to explicitly define a delegate that encapsulates a parameterless method. For example, the following code explicitly declares a delegate named WriteMethod and assigns a reference to the OutputTarget.SendToFile instance method to its delegate instance.
Imports System.IO
Delegate Function WriteMethod As Boolean
Module TestDelegate
Public Sub Main()
Dim output As New OutputTarget()
Dim methodCall As WriteMethod = AddressOf output.SendToFile
If methodCall() Then
Console.WriteLine("Success!")
Else
Console.WriteLine("File write operation failed.")
End If
End Sub
End Module
Public Class OutputTarget
Public Function SendToFile() As Boolean
Try
Dim fn As String = Path.GetTempFileName
Dim sw As StreamWriter = New StreamWriter(fn)
sw.WriteLine("Hello, World!")
sw.Close
Return True
Catch
Return False
End Try
End Function
End Class
using System;
using System.IO;
delegate bool WriteMethod();
public class TestDelegate
{
public static void Main()
{
OutputTarget output = new OutputTarget();
WriteMethod methodCall = output.SendToFile;
if (methodCall())
Console.WriteLine("Success!");
else
Console.WriteLine("File write operation failed.");
}
}
public class OutputTarget
{
public bool SendToFile()
{
try
{
string fn = Path.GetTempFileName();
StreamWriter sw = new StreamWriter(fn);
sw.WriteLine("Hello, World!");
sw.Close();
return true;
}
catch
{
return false;
}
}
}
The following example simplifies this code by instantiating the Func<(Of <(TResult>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
Imports System.IO
Module TestDelegate
Public Sub Main()
Dim output As New OutputTarget()
Dim methodCall As Func(Of Boolean) = AddressOf output.SendToFile
If methodCall() Then
Console.WriteLine("Success!")
Else
Console.WriteLine("File write operation failed.")
End If
End Sub
End Module
Public Class OutputTarget
Public Function SendToFile() As Boolean
Try
Dim fn As String = Path.GetTempFileName
Dim sw As StreamWriter = New StreamWriter(fn)
sw.WriteLine("Hello, World!")
sw.Close
Return True
Catch
Return False
End Try
End Function
End Class
using System;
using System.IO;
public class TestDelegate
{
public static void Main()
{
OutputTarget output = new OutputTarget();
Func<bool> methodCall = output.SendToFile;
if (methodCall())
Console.WriteLine("Success!");
else
Console.WriteLine("File write operation failed.");
}
}
public class OutputTarget
{
public bool SendToFile()
{
try
{
string fn = Path.GetTempFileName();
StreamWriter sw = new StreamWriter(fn);
sw.WriteLine("Hello, World!");
sw.Close();
return true;
}
catch
{
return false;
}
}
}
You can use the Func<(Of <(TResult>)>) delegate with anonymous methods in C#, as the following example illustrates. (For an introduction to anonymous methods, see Anonymous Methods (C# Programming Guide).)
using System;
using System.IO;
public class Anonymous
{
public static void Main()
{
OutputTarget output = new OutputTarget();
Func<bool> methodCall = delegate() { return output.SendToFile(); };
if (methodCall())
Console.WriteLine("Success!");
else
Console.WriteLine("File write operation failed.");
}
}
public class OutputTarget
{
public bool SendToFile()
{
try
{
string fn = Path.GetTempFileName();
StreamWriter sw = new StreamWriter(fn);
sw.WriteLine("Hello, World!");
sw.Close();
return true;
}
catch
{
return false;
}
}
}
You can also assign a lambda expression to a Func<(Of <(T, TResult>)>) delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions (C# Programming Guide).)
Imports System.IO
Module TestDelegate
Public Sub Main()
Dim output As New OutputTarget()
Dim methodCall As Func(Of Boolean) = Function() output.SendToFile()
If methodCall() Then
Console.WriteLine("Success!")
Else
Console.WriteLine("File write operation failed.")
End If
End Sub
End Module
Public Class OutputTarget
Public Function SendToFile() As Boolean
Try
Dim fn As String = Path.GetTempFileName
Dim sw As StreamWriter = New StreamWriter(fn)
sw.WriteLine("Hello, World!")
sw.Close
Return True
Catch
Return False
End Try
End Function
End Class
using System;
using System.IO;
public class Anonymous
{
public static void Main()
{
OutputTarget output = new OutputTarget();
Func<bool> methodCall = () => output.SendToFile();
if (methodCall())
Console.WriteLine("Success!");
else
Console.WriteLine("File write operation failed.");
}
}
public class OutputTarget
{
public bool SendToFile()
{
try
{
string fn = Path.GetTempFileName();
StreamWriter sw = new StreamWriter(fn);
sw.WriteLine("Hello, World!");
sw.Close();
return true;
}
catch
{
return false;
}
}
}
The underlying type of a lambda expression is one of the generic Func delegates. This makes it possible to pass a lambda expression as a parameter without explicitly assigning it to a delegate. In particular, because many methods of types in the System.Linq namespace have Func parameters, you can pass these methods a lambda expression without explicitly instantiating a Func delegate.
If you have an expensive computation that you want to execute only if the result is actually needed, you can assign the expensive function to a Func<(Of <(TResult>)>) delegate. The execution of the function can then be delayed until a property that accesses the lazy value is used in an expression. The example in the next section demonstrates how to do this.