Get class from Type with Reflection and call a generic constructor with Type in C#

Question

I am working with Dapper and I want to iterate through my model classes and set type mapping for any class having a field decorated with a ColumnAttribute.

public class ColumnAttributeTypeMapper<T> : FallbackTypeMapper
{
    public static readonly string ColumnAttributeName = "ColumnAttribute";

    public ColumnAttributeTypeMapper()
        : base(new SqlMapper.ITypeMap[]
        {
            new CustomPropertyTypeMap(typeof (T), SelectProperty),
            new DefaultTypeMap(typeof (T))
        })
    {
    }
    // implementation of SelectProperty and so on...
    // If required, full implementation is on https://gist.github.com/senjacob/8539127
}

In my model class library, I'm iterating through all possible types; now I need to call the generic ColumnAttributeTypeMapper<T> constructor with the class of the type.

using System.Web;
using Dapper;

[assembly : PreApplicationStartMethod(typeof(Model.Initiator), "RegisterTypeMaps")]

namespace Model
{
    class Initiator
    {
        public static void RegisterTypeMaps()
        {
            var mappedTypes = Assembly.GetAssembly(typeof (Initiator)).GetTypes().Where(
                f =>
                f.GetProperties().Any(
                    p =>
                    p.GetCustomAttributes(false).Any(
                        a => a.GetType().Name == ColumnAttributeTypeMapper<dynamic>.ColumnAttributeName)));

            // I want to skip registering each class manually :P
            // SqlMapper.SetTypeMap(typeof(Model1), new ColumnAttributeTypeMapper<Model1>());
            // SqlMapper.SetTypeMap(typeof(Model2), new ColumnAttributeTypeMapper<Model2>());

            foreach (var mappedType in mappedTypes)
            {
                SqlMapper.SetTypeMap(mappedType, new ColumnAttributeTypeMapper<mappedType>());
            }
        }
    }
}

How can I pass the class from type instead of type 'mappedType' to new ColumnAttributeTypeMapper<classof(mappedType)?>()

I found this as a similar question, but I need to call the generic constructor instead of a generic method with the Type.

If it can not be done, could you please explain the reason?

Answer

This is how the mapping worked as suggested by Tom.

var mapper = typeof(ColumnAttributeTypeMapper<>);
foreach (var mappedType in mappedTypes)
{
    var genericType = mapper.MakeGenericType(new[] { mappedType });
    SqlMapper.SetTypeMap(mappedType, Activator.CreateInstance(genericType) as SqlMapper.ITypeMap);
}

[C sharp dynamic generic type](http://stackoverflow.com/questions/2078914/c-sharp-dynamic-generic-type) | [Dynamically create a generic type in C#](http://stackoverflow.com/questions/67370/dynamically-create-a-generic-type-for-template) — Sen Jacob, Jan 21 '14 at 18:41

score 3 · Accepted Answer · answered Jan 21 '14 at 17:11

You will need the method Type.MakeGenericType; usage is as follows:

var columnType = typeof(ColumnAttributeTypeMapper<>);
var genericColumn = columnType.MakeGenericType(new[] {typeof(mappedType)});
var instance = Activator.CreateInstance(genericColumn);

I'm writing this without intellisense and only having skimmed your code, so please let me know whether I've made any mistakes and I'll correct them.

Get class from Type with Reflection and call a generic constructor with Type in C#

Answer

1 Answers1