构建LINQ EX pression树：如何让变量的作用域 [英] Building a LINQ expression tree: how to get variable in scope

问题描述

我要建一个LINQ EX pression树，但它不会编译，因为涉嫌局部变量 $ VAR1 超出范围：

I'm building a LINQ expression tree but it won't compile because allegedly the local variable $var1 is out of scope:

变量'''从范围引用的，但它没有定义System.Object的'类型'的

这是前pression树：

This is the expression tree:

.Block() {
    $var1;
    .If ($n.Property1 == null) {
        .Block() {
            $var1 = null;
            .Return #Label1 { }
        }
    } .Else {
        .Default(System.Void)
    };
    $var1 = (System.Object)($n.Property1).Length;
    .Label
    .LabelTarget #Label1:;
    $var1
}

下面code是负责建设的树。这是一个更大的东西的一部分，所以我不希望它的目的是从这个例子非常清楚。

The following code is responsible for building the tree. It is part of something larger, therefore I don't expect its purpose to be perfectly clear from this example.

MemberExpression sourceExpression = ...;

List<Expression> expressions = new List<Expression>();
LabelTarget returnTarget = Expression.Label();
ParameterExpression resultVariable = Expression.Variable(typeof(object));

expressions.Add(resultVariable);

expressions.Add(
    Expression.IfThen(
        Expression.Equal(sourceExpression, Expression.Constant(null)),
        Expression.Block(
            Expression.Assign(resultVariable, Expression.Constant(null)),
            Expression.Return(returnTarget))));

expressions.Add(
    Expression.Assign(
        resultVariable,
        Expression.Convert(sourceExpression, typeof(object))));

expressions.Add(Expression.Label(returnTarget));
expressions.Add(resultVariable);

Expression finalExpression = Expression.Block(expressions);
object result = Expression.Lambda<Func<object>>(finalExpression).Compile()();

所以，问题是：我如何才能在局部变量为范围，以使前pression编译成功地

So the question is: how do I get the local variable into scope so that the expression compiles succesfully?

推荐答案

傻冒添加防爆pression.Variable 正常的名单前pressions块中的 - 你应该使用过载，它指定的变量也分别申报块< /一>：

Your'e adding the Expression.Variable to the list of "normal" expressions in the block - you should use the overload which specifies the variables do declare for the block separately:

Expression finalExpression = Expression.Block(new[] { resultVariable },
                                              expressions);

（并删除调用 EX pressions.Add（resultVariable）; ）

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