C# 0 The Complete Reference

Compile( )

method defined by 

Expression

. It returns a reference that can be assigned to a delegate and 

then executed. You can declare your own delegate type or use one of the predefined 

Func

delegate types defined within the 

System

 namespace. Two forms of the 

Func

 delegate were 

mentioned earlier, when the query methods were described. Here is a list of all its forms:

delegate TResult Func( )

delegate TResult Func( )

delegate TResult Func( )

delegate TResult Func( )

delegate TResult Func( )

These forms represent methods that return a value and take from zero to four parameters 

(whose types are 

T1

 through 

T4

). If your expression requires more than four parameters, 

then you will need to define your own delegate type.

Expression trees have one key restriction: Only expression lambdas can be represented 

by expression trees. They cannot be used to represent statement lambdas.

Here is a simple example of an expression tree in action. It creates an expression tree 

whose data represents a method that determines if one integer is a factor of another. It then 

compiles the expression tree into executable code. Finally, it demonstrates the compiled code.

// A simple expression tree.

using System;

using System.Linq;

using System.Linq.Expressions;

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class SimpleExpTree {

  static void Main() {

    // Represent a lambda expression as data.

    Expression>

      IsFactorExp = (n, d) => (d != 0) ? (n % d) == 0 : false;

    // Compile the expression data into executable code.

    Func IsFactor = IsFactorExp.Compile();

    // Execute the expression.

    if(IsFactor(10, 5))

      Console.WriteLine("5 is a factor of 10.");

    if(!IsFactor(10, 7))

      Console.WriteLine("7 is not a factor of 10.");

    Console.WriteLine();

  }

}

The output is shown here:

5 is a factor of 10.

7 is not a factor of 10.

The program illustrates the two key steps in using an expression tree. First, it creates an 

expression tree by using this statement:

Expression>

  IsFactorExp = (n, d) => (d != 0) ? (n % d) == 0 : false;

This constructs a representation of a lambda expression in memory. As explained, this 

representation is data, not code. This representation is referred to by 

IsFactorExp

. The 

following statement converts the expression data into executable code:

Func IsFactor = IsFactorExp.Compile();

After this statement executes, the 

IsFactor

 delegate can be called to determine if one value is 

a factor of another.

One other point: Notice that