Question

尝试使用表达式树创建以下表达式（希望得到帮助）

List<string> lstName = dt_Name.Select(y => y.Name);

List<string> lstLabName = dt_Label.Select(x => x.LabelName).Where(p => p.LabelName.StartsWith(lstName + '_'));

我希望它实际上遍历所有lstName并找到符合条件的lstLabName中的所有实例。

到目前为止我提出的代码是：

    private BinaryExpression CreateBinaryExpression(string buildName)
    {
        // x.LabelName
        var param = Expression.Parameter(typeof(dt_Label), "x");
        var key = param.GetType().GetProperty("LabelName");
        var left = Expression.MakeMemberAccess(param, key);

        //ParameterExpression left = Expression.Parameter(typeof(string), "p"); //p
        ConstantExpression constantExpression = Expression.Constant(buildName, typeof(string));

        // x.LabelName == buildName
        return Expression.MakeBinary(ExpressionType.Equal, left, constantExpression);
    }

    private BinaryExpression CalculateLambdaExp(List<string> lstBuildName) 
    {
            BinaryExpression binaryExpression = CreateBinaryExpression(lstBuildName[0]);

            if (lstBuildName.Count() > 1)
            {
                List<string> lstnStr = lstBuildName;
                lstnStr.RemoveAt(0);
                BinaryExpression calculatedLambdaExp = CalculateLambdaExp(lstnStr);
                binaryExpression = Expression.MakeBinary(ExpressionType.AndAlso, binaryExpression, calculatedLambdaExp);
            }

            return binaryExpression;
    }

    private List<string> RunLambdaExpression(List<string> BuildName)
    {
        ParameterExpression left = Expression.Parameter(typeof(string), "p"); //p

        var factorial = Expression.Lambda<Func<List<string>, List<string>>>(CalculateLambdaExp(BuildName), left);
        var n = factorial.Compile()(BuildName);
        List<string> lst = n.ToList();

        return lst;
    }

我遇到了大量的运行时错误。我将不胜感激。

计算了大部分内容：

将CreateBinaryExpression函数更改为：

    private Expression CreateBinaryExpression(string buildName)
    {
        // x.LabelName
        var param = Expression.Parameter(typeof(dt_Label), "x");
        var key = typeof(dt_Label).GetProperty("LabelName");

        var left = Expression.MakeMemberAccess(param, key);

        ConstantExpression constantExpression = Expression.Constant(buildName + '_', typeof(string));

        //x.LabelName.startsWith(buildName_)
        MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
        Expression call = Expression.Call(left, mi, constantExpression);

        return call;
    };

并更改了对函数的所有引用以接收Expression而不是BinaryExpression

Answer 1

这一行是个问题

var key = param.GetType().GetProperty("LabelName");

param.GetType()将返回没有System.Linq.Expressions.ParameterExpression属性的LabelName。

您应该使用typeof(dt_Label).GetProperty("LabelName")

无法构建lambda表达式树

1 个答案: