Dynamic Expression Generation Issues with ValueTypes

Question

I built a framework that allows for cascaded sorting of report data in a table depending on what column is the master sorted column. It works for the most part, except in one specific, but important case: when the field's property is a value type. I receive the following error message:

System.ArgumentException: Expression of type 'System.Int32' cannot be used for return type 'System.Object'

I know that this means I need to box the value of the ValueType, but I'm not completely sure how to in this particular situation. Per some research and this SO answer I believe that I need to use Expression.Convert in some way.

My code below is what generates the expressions. The generic type parameter T is the type of the "row" of data. The GetFullSortOrder() simply returns an array of strings that represent the names of the columns (properties) in the type T that will also be sorted.

public IEnumerable<Expression<Func<T, object>>> GetExpressions<T>(string sortedColumn) where T : IReportRecord
    {
        var columns = GetFullSortOrder(sortedColumn)
        var typeParameter = Expression.Parameter(typeof(T));
        foreach (var c in columns)
        {
            var propParameter = Expression.Property(typeParameter, c);
            yield return Expression.Lambda<Func<T, object>>(propParameter, typeParameter);
        }
    }

The exception is thrown when processing Expression.Lambda<Func<T, object>>() when the Property selected in T is of a ValueType. What is needed to property box or return the correct value when the types aren't known until run-time?

score 3 · Accepted Answer · answered Feb 03 '17 at 20:40

You said it - you need to use Expression.Convert and pass typeof(object). If you want to simulate what the C# compiler does, you should do it only for value types:

Expression result = propParameter;
if (typeof(T).IsValueType)
    result = Expression.Convert(result, typeof(object));
yield return Expression.Lambda<Func<T, object>>(result, typeParameter);

Maksim Simkin · Answer 2 · 2017-02-03T20:43:49.997

1

You have to convert your property Expression to an object type:

var propParameterObj = Expression.Convert(propParameter, typeof(object));

edited Feb 03 '17 at 20:43

answered Feb 03 '17 at 20:34

Maksim Simkin

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And then use that in the `Expression.Lambda<>()` call? – JNYRanger Feb 03 '17 at 20:35
@JNYRanger yes, i would do it for every parameter, but perhaps converting for ref types is not neccessery – Maksim Simkin Feb 03 '17 at 20:36
Yeah It's def not necessary for reference types, but I'll optimize it later on once things are working for all types. – JNYRanger Feb 03 '17 at 20:37

Dynamic Expression Generation Issues with ValueTypes

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