show initial type-handler design in line with DapperLib/Dapper#1924

src/Dapper.AOT/CommandFactory.cs

@@ -102,6 +102,21 @@ protected static object AsValue<T>(T? value) where T : struct
     protected static object AsValue(object? value)
         => value ?? DBNull.Value;
 
+
+    /// <summary>
+    /// Wrap a value for use in an ADO.NET parameter
+    /// </summary>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    internal static object AsGenericValue<T>(T value)
+    {
+        // note JIT elide here
+        if (typeof(T) == typeof(bool)) return AsValue(Unsafe.As<T, bool>(ref value));
+        if (typeof(T) == typeof(bool?)) return AsValue(Unsafe.As<T, bool?>(ref value));
+        if (typeof(T) == typeof(int)) return AsValue(Unsafe.As<T, int>(ref value));
+        if (typeof(T) == typeof(int?)) return AsValue(Unsafe.As<T, int?>(ref value));
+        return AsValue((object?)value);
+    }
+
     /// <summary>
     /// Flexibly parse an <see cref="object"/> as a value of type <typeparamref name="T"/>.
     /// </summary>

src/Dapper.AOT/TypeHandlerT.cs

@@ -0,0 +1,34 @@
+using Dapper.Internal;
+using System;
+using System.ComponentModel;
+using System.Data.Common;
+
+namespace Dapper;
+
+/// <summary>
+/// Specify that <typeparamref name="TTypeHandler"/> should be used as a custom type-handler
+/// when processing values of type <typeparamref name="TValue"/>
+/// </summary>
+[ImmutableObject(true)]
+[AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+public sealed class TypeHandlerAttribute<TValue, TTypeHandler> : Attribute
+    where TTypeHandler : TypeHandler<TValue>, new()
+{}
+
+/// <summary>
+/// Process a parameter value of type <typeparamref name="T"/>
+/// </summary>
+public abstract class TypeHandler<T>
+{
+    /// <summary>
+    /// Configure and assign a value for a parameter
+    /// </summary>
+    public virtual void SetValue(DbParameter parameter, T value)
+        => parameter.Value = CommandFactory.AsGenericValue<T>(value);
+
+    /// <summary>
+    /// Interpret the output value from a parameter
+    /// </summary>
+    public virtual T Parse(DbParameter parameter)
+        => CommandUtils.As<T>(parameter.Value);
+}