CODEWITHSUNDEEP
c#reflectionlambdaexpression-trees
Morten Holmgaard
Morten Holmgaard

Reputation: 7806

Building expression tree with LINQ Min method - throws ArgumentException

using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace Test_console_application
{
    class Program
    {
        static void Main(string[] args)
        {
            var parentPropertyName = "Measurements";
            var parentPropertyType = typeof (Measurement);
            var propertyName = "Data";

            var parameterExp = Expression.Parameter(typeof(Inverter), "type");
            var propertyExp = Expression.Property(parameterExp, parentPropertyName);

            var method = typeof(Enumerable).GetMethods(BindingFlags.Static | BindingFlags.Public)
                .Single(x => x.ToString() == "Double Min[TSource](System.Collections.Generic.IEnumerable`1[TSource], System.Func`2[TSource,System.Double])")
                .MakeGenericMethod(parentPropertyType);

            var minParameterExp = Expression.Parameter(parentPropertyType, "type2");
            var minPropertyExp = Expression.Property(minParameterExp, propertyName);
            var minMethodExp = Expression.Call(method, propertyExp, minPropertyExp);            
        }
    }

    public class Inverter
    {
        public IList<Measurement> Measurements { get; set; }
    }

    public class Measurement
    {
        public double Data { get; set; }
    }
}

When I run this code I get a ArgumentException:

Expression of type 'System.Double' cannot be used for parameter of type 'System.Func2[Test_console_application.Measurement,System.Double]' of method 'Double Min[Measurement](System.Collections.Generic.IEnumerable1[Test_console_application.Measurement], System.Func`2[Test_console_application.Measurement,System.Double])'

I understands what it says, but I just thought that I was what I was doing that with minPropertyExp.
I can't figure out what I need to change - any clue?

Upvotes: 1

Views: 626

Answers (1)

Stefan Steinegger
Stefan Steinegger

Reputation: 64648

You should not pass a property expression as Func. You should pass a method.

You did something like:

Measurements.Min(type2.Data)

Instead of

Measurements.Min(x => x.Data)

From the comment by the Morten Holmgaard

var minMethod = Expression.Lambda(minPropertyExp, minParameterExp);
var minMethodExp = Expression.Call(method, propertyExp, minMethod);

Upvotes: 1

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