How can I use an expression tree to call a generic method when the Type is only known at runtime?

Question

This is something that I solved using reflection, but would like to see how to do it using expression trees.

I have a generic function:

private void DoSomeThing<T>( param object[] args ) {
    // Some work is done here.
}

that I need to call from else where in my class. Now, normally, this would be be simple:

DoSomeThing<int>( blah );

but only if I know, at design time that I am working with an int. When I do not know the type until runtime is where I need the help. Like I said, I know how to do it via reflection, but I would like to do it via expression trees, as my (very limited) understanding is that I can do so.

Any suggestions or points to sites where I can get this understanding, preferably with sample code?

I don't understand how this question *explicitly asked about expression trees* yet the answer that was accepted *says nothing about them*. Ben's answer, although far later, actually answers the question asked **and** is far more complete to boot. — Ian Kemp, Jul 16 '21 at 09:25

score 11 · Answer 1 · answered Sep 12 '14 at 16:34

Yes, it can be done via expression trees. The advantage is that you get a delegate so repeated calls will be far faster than doing MethodInfo.Invoke() over and over again. The dynamic keyword can do this also.

Example:

What type would you like to use?
decimal
Selected type 'System.Decimal'
Input Value:
5.47
<<<USING object>>>
The object has static type 'System.Object',  dynamic type 'System.Decimal', and value '5.47'
<<<USING dynamic>>>
The object has static type 'System.Decimal',  dynamic type 'System.Decimal', and value '5.47'
<<<USING reflection>>>
The object has static type 'System.Decimal',  dynamic type 'System.Decimal', and value '5.47'
<<<USING expression tree>>>
The object has static type 'System.Decimal',  dynamic type 'System.Decimal', and value '5.47'

Code:

using System;
using System.ComponentModel;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace SO2433436
{
    class Program
    {
        static void LogObject<T>(T t)
        {
            Console.WriteLine("The object has static type '" + typeof(T).FullName + "',  dynamic type '" + t.GetType() + "', and value '" + t.ToString() + "'");
        }

        static void Main(string[] args)
        {
            Console.WriteLine("What type would you like to use?");
            string typeName = Console.ReadLine();

            Type userType;
            switch (typeName)
            {
                case "byte": userType = typeof(byte); break;
                case "sbyte": userType = typeof(sbyte); break;
                case "ushort": userType = typeof(ushort); break;
                case "short": userType = typeof(short); break;
                case "uint": userType = typeof(uint); break;
                case "int": userType = typeof(int); break;
                case "string": userType = typeof(string); break;
                case "decimal": userType = typeof(decimal); break;
                default:
                    userType = Type.GetType(typeName);
                    break;
            }

            Console.WriteLine("Selected type '" + userType.ToString() + "'");

            Console.WriteLine("Input Value:");
            string val = Console.ReadLine();

            object o = TypeDescriptor.GetConverter(userType).ConvertFrom(val);

            Console.WriteLine("<<<USING object>>>");
            LogObject(o);

            Console.WriteLine("<<<USING dynamic>>>");
            LogObject((dynamic)o);

            Console.WriteLine("<<<USING reflection>>>");
            Action<object> f = LogObject<object>;
            MethodInfo logger = f.Method.GetGenericMethodDefinition().MakeGenericMethod(userType);
            logger.Invoke(null, new[] { o });

            Console.WriteLine("<<<USING expression tree>>>");
            var p = new[] { Expression.Parameter(typeof(object)) };
            Expression<Action<object>> e =
                Expression.Lambda<Action<object>>(
                    Expression.Call(null,
                                    logger,
                                    Expression.Convert(p[0], userType)
                                   )
                , p);
            Action<object> a = e.Compile();
            a(o);
        }
    }
}

leppie · Accepted Answer · 2010-03-12T15:44:11.147

7

MethodInfo.MakeGenericMethod

~~Then just create a delegate and call it.~~ (not in an expression, of course ;p)

Update:

Generally, I prefer to use generic types for this, Activator.CreateInstance just requires less work. All depends on your situation though.

edited Mar 12 '10 at 15:44

answered Mar 12 '10 at 14:54

leppie

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1

As I stated, I know how to do it via reflection. I am attempting to do in via expression trees. – David Williams Mar 12 '10 at 14:57
1

Just the same, isn't it? Am I missing something? (Updated answer) – leppie Mar 12 '10 at 15:42

How can I use an expression tree to call a generic method when the Type is only known at runtime?

2 Answers2

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