从表达式中获取结果

尝试用编译表达式 Compile 方法,然后调用返回的委托:

using System;
using System.Linq.Expressions;

class Example
{
    static void Main()
    {
        Expression<Func<Boolean>> expression 
                = () => "MyValue".StartsWith("MyV");
        Func<Boolean> func = expression.Compile();
        Boolean result = func();
    }
}

正如安德鲁提到的,在执行表达式之前必须先编译它。另一种选择是根本不使用表达式,这样做会:

Func<Boolean> MyLambda = () => "MyValue".StartsWith("MyV");
var Result = MyLambda();

在本例中,lambda表达式是在构建项目时编译的,而不是转换为表达式树。如果您没有动态地操作表达式树或使用使用表达式树(Linq to SQL、Linq to Entities等)的库,那么这样做更有意义。

我要做的就是从这里开始: MSDN example

delegate int del(int i);
static void Main(string[] args)
{
    del myDelegate = x => x * x;
    int j = myDelegate(5); //j = 25
}

如果您想使用 Expression<TDelegate> 然后键入此页: Expression(TDelegate) Class (System.Linq.Expression) 有以下示例:

// Lambda expression as executable code.
Func<int, bool> deleg = i => i < 5;
// Invoke the delegate and display the output.
Console.WriteLine("deleg(4) = {0}", deleg(4));

// Lambda expression as data in the form of an expression tree.
System.Linq.Expressions.Expression<Func<int, bool>> expr = i => i < 5;
// Compile the expression tree into executable code.
Func<int, bool> deleg2 = expr.Compile();
// Invoke the method and print the output.
Console.WriteLine("deleg2(4) = {0}", deleg2(4));

/*  This code produces the following output:
    deleg(4) = True
    deleg2(4) = True
*/