表达式：使用API创建表达式树（2）

//声明一个包含有4个表达式的表达式块。
BlockExpression blockExpr = Expression.Block(Expression.Call(null,
typeof(Console).GetMethod("Write", new Type[] { typeof(String) }), Expression.Constant("你好！")),// 1
Expression.Call(null, typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
Expression.Constant("BlockExpression表达式!")), // 2
Expression.Constant(42), //3
Expression.Call(null, typeof(string).GetMethod("Concat", new Type[] { typeof(string), typeof(string) }),
Expression.Constant("测试1 "), Expression.Constant("测试2")) //4
);
Console.WriteLine("***********************");
Console.WriteLine("显示表达示块的执行结果:");
// 首先创建表达式树,
// 编译，执行.
var result = Expression.Lambdastring>>(blockExpr).Compile()();
// 显示表达式块各表达式.
Console.WriteLine("表达式块中的各表达式:");
foreach (var expr in blockExpr.Expressions)
Console.WriteLine(expr.ToString());
// 表达式返回值
Console.WriteLine("表达式返回值");
Console.WriteLine(result);
//***********************
// 显示表达示块的执行结果:
//你好！BlockExpression表达式!
//表达式块中的各表达式:
//Write("你好！")
//WriteLine("BlockExpression表达式!")
//42
//Concat("测试1 ", "测试2")
//表达式返回值
//测试1 测试2

TryExpression tryCatchExpr = Expression.TryCatch(
Expression.Block(
Expression.Divide(Expression.Constant(50), Expression.Constant(2)),
Expression.Constant("无异常返回")
),
Expression.Catch(
typeof(DivideByZeroException),
Expression.Constant("异常返回")
)
);
Console.WriteLine(Expression.Lambdastring>>(tryCatchExpr).Compile()());
//输出为 ：无异常返回

//输出为：异常返回

TryExpression tryCatchExpr2 = Expression.TryCatchFinally(
Expression.Block(
Expression.Divide(Expression.Constant(50), Expression.Constant(2)),
Expression.Constant("无导常返回")
),
Expression.Call(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) }),
　　　　Expression.Constant("Finally 块")),
Expression.Catch(typeof(Exception),
Expression.Constant("导常返回")
)
);
Console.WriteLine(Expression.Lambdastring>>(tryCatchExpr2).Compile()());