解析入口參數(shù)為實(shí)體的表達(dá)式樹

在學(xué)習(xí)LINQ的時(shí)候碰到了解析表達(dá)式樹的問題，書上的例子中入口參數(shù)為一個(gè)單一變量，這個(gè)在MSDN中給出了示例代碼：

MSDN官方示例：

// Add the following using directive to your code file:
// using System.Linq.Expressions;
// Create an expression tree.
Expression<Func<int, bool>> exprTree = num => num <5;
// Decompose the expression tree.
ParameterExpression param = (ParameterExpression)exprTree.Parameters[0];
BinaryExpression operation = (BinaryExpression)exprTree.Body;
ParameterExpression left = (ParameterExpression)operation.Left;
ConstantExpression right = (ConstantExpression)operation.Right;
Console.WriteLine("Decomposed expression: {0} => {1} {2} {3}",
param.Name, left.Name, operation.NodeType, right.Value);
// This code produces the following output:
// Decomposed expression: num => num LessThan 5

這里表達(dá)式的入口參數(shù)是一個(gè)int型的變量，表達(dá)式左邊就是這個(gè)參數(shù)本身，右邊是一個(gè)常量，因此用

ParameterExpression left = (ParameterExpression)operation.Left;

ConstantExpression right = (ConstantExpression)operation.Right;

來解析即可，但如果入口是一個(gè)實(shí)體呢，這種情況很常見，應(yīng)該怎么解析呢，通過向高人請教，最后用編譯表達(dá)式樹終于解決了這個(gè)問題：

    //實(shí)體類
    public class T
    {
        public int ID { get; set; }
        public string Name { get; set; }
    }

    //解析表達(dá)式樹類
    public class Test
    {
        public void ExpressionTest(Expression<Func<T, bool>> exprTree)
        {
            ParameterExpression param = (ParameterExpression)exprTree.Parameters[0];
            BinaryExpression operation = (BinaryExpression)exprTree.Body;
            MemberExpression left = (MemberExpression)operation.Left;
            MemberExpression right = (MemberExpression)operation.Right;

            var result=Expression.Lambda(right).Compile().DynamicInvoke();
            Console.WriteLine("Decomposed expression:{0}=>{1} {2} {3}",
                param.Name,left.Member.Name,operation.NodeType,result.ToString());
        }
    }

    //調(diào)用
    public class Program
    {
        static void Main(string[] args)
        {
            Test test = new Test();
            T t = new T() { ID = 1, Name = "abc" };
            test.ExpressionTest(o => o.ID == t.ID);
            test.ExpressionTest(o => o.Name == t.Name);
            Console.Read();
        }
    }

運(yùn)行結(jié)果如下：

園子中有位朋友提供了另外一種方法（http://home.cnblogs.com/q/22976/），不過那個(gè)方法有一定缺陷，要依賴于實(shí)體類型，如果像上面代碼中那樣把表達(dá)式做為一個(gè)參數(shù)就無能為力了。

posted @ 2011-03-15 23:35 artwl 閱讀(520) 評論(1) 收藏舉報(bào)

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解析入口參數(shù)為實(shí)體的表達(dá)式樹

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