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 three parameters, each of which is passed to it by value, and that it must return a value.
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To reference a method that has three parameters and returns
void (or in Visual Basic, that is declared as a Sub rather than as a Function), use the generic Action<(Of <(T1, T2, T3>)>) delegate instead.
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When you use the Func<(Of <(T1, T2, T3, TResult>)>) delegate, you do not have to explicitly define a delegate that encapsulates a method with three parameters. For example, the following code explicitly declares a generic delegate named ParseNumber and assigns a reference to the Parse method to its delegate instance.
Imports System.Globalization
Delegate Function ParseNumber(Of T)(input As String, styles As NumberStyles, _
provider As IFormatProvider) As T
Module DelegateExample
Public Sub Main()
Dim numericString As String = "-1,234"
Dim parser As ParseNumber(Of Integer) = AddressOf Integer.Parse
Console.WriteLine(parser(numericString, _
NumberStyles.Integer Or NumberStyles.AllowThousands, _
CultureInfo.InvariantCulture))
End Sub
End Module
using System;
using System.Globalization;
delegate T ParseNumber<T>(string input, NumberStyles styles,
IFormatProvider provider);
public class DelegateExample
{
public static void Main()
{
string numericString = "-1,234";
ParseNumber<int> parser = int.Parse;
Console.WriteLine(parser(numericString,
NumberStyles.Integer | NumberStyles.AllowThousands,
CultureInfo.InvariantCulture));
}
}
The following example simplifies this code by instantiating the Func<(Of <(T1, T2, T3, TResult>)>) delegate rather than explicitly defining a new delegate and assigning a named method to it.
Imports System.Globalization
Module GenericFunc
Public Sub Main()
Dim numericString As String = "-1,234"
Dim parser As Func(Of String, NumberStyles, IFormatProvider, Integer) _
= AddressOf Integer.Parse
Console.WriteLine(parser(numericString, _
NumberStyles.Integer Or NumberStyles.AllowThousands, _
CultureInfo.InvariantCulture))
End Sub
End Module
using System;
using System.Globalization;
public class GenericFunc
{
public static void Main()
{
string numericString = "-1,234";
Func<string, NumberStyles, IFormatProvider, int> parser = int.Parse;
Console.WriteLine(parser(numericString,
NumberStyles.Integer | NumberStyles.AllowThousands,
CultureInfo.InvariantCulture));
}
}
You can use the Func<(Of <(T1, T2, T3, 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.Globalization;
public class Anonymous
{
public static void Main()
{
string numericString = "-1,234";
Func<string, NumberStyles, IFormatProvider, int> parser =
delegate(string s, NumberStyles sty, IFormatProvider p)
{ return int.Parse(s, sty, p); };
Console.WriteLine(parser(numericString,
NumberStyles.Integer | NumberStyles.AllowThousands,
CultureInfo.InvariantCulture));
}
}
You can also assign a lambda expression to a Func<(Of <(T1, T2, T3, TResult>)>) delegate, as the following example illustrates. (For an introduction to lambda expressions, see Lambda Expressions and Lambda Expressions (C# Programming Guide).)
Imports System.Globalization
Module LambdaExpression
Public Sub Main()
Dim numericString As String = "-1,234"
Dim parser As Func(Of String, NumberStyles, IFormatProvider, Integer) _
= Function(s, sty, p) Integer.Parse(s, sty, p)
Console.WriteLine(parser(numericString, _
NumberStyles.Integer Or NumberStyles.AllowThousands, _
CultureInfo.InvariantCulture))
End Sub
End Module
using System;
using System.Globalization;
public class LambdaExpression
{
public static void Main()
{
string numericString = "-1,234";
Func<string, NumberStyles, IFormatProvider, int> parser = (s, sty, p)
=> int.Parse(s, sty, p);
Console.WriteLine(parser(numericString,
NumberStyles.Integer | NumberStyles.AllowThousands,
CultureInfo.InvariantCulture));
}
}
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.