Question

我在旋转我构建的表达式树生成器时遇到了一些麻烦。

本质上，我想将以下内容转换为表达式树：

var res = myObjInstance.AnalysisResponseMeasures
           .First(p => p.MeasureTypeId == 1).MeasureValue

到目前为止，我已经：

var propertyExp =  Expression.Property(pe, "AnalysisResponseMeasures");
            var someParam = Expression.Parameter(typeof(AnalysisResponseMeasure), "p");
            var someParam2 = Expression.Parameter(typeof(AnalysisResponseMeasure), "p");
            var childPropExp = Expression.Equal(
                     Expression.Property(someParam, "MeasureTypeId"),
                     Expression.Constant(1));

            var firstExp = Expression.Call(
            typeof(IEnumerable<AnalysisResponseMeasure>),
            "First",
            new[] { typeof(AnalysisResponseMeasure) },
            propertyExp,
            Expression.Lambda(childPropExp, someParam2));

            var selectExp = Expression.Property(firstExp, "MeasureValue");

但是，我收到运行时错误，指出IEnumerable<AnalysisResponseMeasure>的类型没有方法First。

我的参数显然是错误的，但我似乎无法弄清楚错误是什么。任何帮助都将非常感激。

Answer 1

玩了一下，也许这适合你：

var p = Expression.Parameter(typeof(AnalysisResponseMeasure), "p");
var prediacte = Expression.Lambda<Func<AnalysisResponseMeasure, bool>>(Expression.Equal(Expression.Property(p, "MeasureTypeId"), Expression.Constant(1)), p);

var firstMethod = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public)
                                    .Single(m => m.Name == "First" &&
                                                 m.GetParameters().Length == 2 &&
                                                 m.GetParameters()[1].ParameterType.IsGenericType &&
                                                 m.GetParameters()[1].ParameterType.GetGenericTypeDefinition() == typeof(Func<,>))
                                    .MakeGenericMethod(typeof(AnalysisResponseMeasure));
var foo = Expression.Parameter(typeof(Foo), "foo");
var first = Expression.Call(firstMethod, Expression.Property(foo, "AnalysisResponseMeasures"), prediacte);
var measureValue = Expression.Property(first, "MeasureValue");
var yourExpression = Expression.Lambda<Func<Foo, double>>(measureValue, foo);

这是不可读的，请确保添加单元测试。

表达式Tree.Call语法

1 个答案: