TypeHandler<T> doesn't seem to be called

c#, dapper, type-conversion

Solution

It isn't you being silly; it is me; the following now pass locally (pushed to github); will deploy to NuGet soon now available on NuGet in v1.27 and above:

public void SO24740733_TestCustomValueHandler()
{
    Dapper.SqlMapper.AddTypeHandler(RatingValueHandler.Default);
    var foo = connection.Query<MyResult>(
        "SELECT 'Foo' AS CategoryName, 200 AS CategoryRating").Single();

    foo.CategoryName.IsEqualTo("Foo");
    foo.CategoryRating.Value.IsEqualTo(200);
}

public void SO24740733_TestCustomValueSingleColumn()
{
    Dapper.SqlMapper.AddTypeHandler(RatingValueHandler.Default);
    var foo = connection.Query<RatingValue>(
        "SELECT 200 AS CategoryRating").Single();

    foo.Value.IsEqualTo(200);
}

Problem

The short version `SqlMapper.Query<T>` seems to ignore my registered `TypeHandler<T>` The long version Here's a simple query: ``` SELECT 'Foo' AS CategoryName, 200 AS CategoryRating ``` ...and here's two POCOs: ``` public class RatingValue { public Int32 Value { get; set; } // ... some other properties etc ... } public class MyResult { public String CategoryName { get; set; } public RatingValue CategoryRating { get; set; } } ``` I've created a new `TypeHandler` implementation which should turn that CategoryRating `Int32` into a `RatingValue` object: ``` public class RatingValueHandler: SqlMapper.TypeHandler<RatingValue> { public override RatingValue Parse(object value) { if (value is Int32) return new RatingValue() { Value = (Int32)value }; throw new FormatException("Invalid conversion to RatingValue"); } public override void SetValue(System.Data.IDbDataParameter parameter, RatingValue value) { // ... null, range checks etc ... parameter.DbType = System.Data.DbType.Int32; parameter.Value = Int32.Parse(value.Value); } } ``` Now, before running my query I'm adding my new handler like this: ``` SqlMapper.AddTypeHandler<RatingValue>(new RatingValueHandler()); ``` However, when I run this: ``` c.Query<MyResult>("SELECT 'Foo' AS CategoryName, 200 AS CategoryRating"); ``` I get an exception from Dapper - it couldn't parse the results in column 1. I expected Dapper to fire my custom type handler! I think I'm missing something very, very obvious. Please tell me how silly I am!

Original source