ColumnAttributeTypeMapper.cs

namespace YourNamespace
{
    /// <summary>
    /// Uses the Name value of the <see cref="ColumnAttribute"/> specified to determine
    /// the association between the name of the column in the query results and the member to
    /// which it will be extracted. If no column mapping is present all members are mapped as
    /// usual.
    /// </summary>
    /// <typeparam name="T">The type of the object that this association between the mapper applies to.</typeparam>
    public class ColumnAttributeTypeMapper<T> : FallbackTypeMapper
    {
        public ColumnAttributeTypeMapper()
            : base(new SqlMapper.ITypeMap[]
                {
                    new CustomPropertyTypeMap(
                       typeof(T),
                       (type, columnName) =>
                           type.GetProperties().FirstOrDefault(prop =>
                               prop.GetCustomAttributes(false)
                                   .OfType<ColumnAttribute>()
                                   .Any(attr => attr.Name == columnName)
                               )
                       ),
                    new DefaultTypeMap(typeof(T))
                })
        {
        }
    }
    
        [AttributeUsage(AttributeTargets.Property, AllowMultiple = true)]
	public class ColumnAttribute : Attribute
	{
		public string Name { get; set; }
	}

  	public class FallbackTypeMapper : SqlMapper.ITypeMap
	{
		private readonly IEnumerable<SqlMapper.ITypeMap> _mappers;

		public FallbackTypeMapper(IEnumerable<SqlMapper.ITypeMap> mappers)
		{
			_mappers = mappers;
		}


		public ConstructorInfo FindConstructor(string[] names, Type[] types)
		{
			foreach (var mapper in _mappers)
			{
				try
				{
					ConstructorInfo result = mapper.FindConstructor(names, types);
					if (result != null)
					{
						return result;
					}
				}
				catch (NotImplementedException)
				{
				}
			}
			return null;
		}

		public SqlMapper.IMemberMap GetConstructorParameter(ConstructorInfo constructor, string columnName)
		{
			foreach (var mapper in _mappers)
			{
				try
				{
					var result = mapper.GetConstructorParameter(constructor, columnName);
					if (result != null)
					{
						return result;
					}
				}
				catch (NotImplementedException)
				{
				}
			}
			return null;
		}

		public SqlMapper.IMemberMap GetMember(string columnName)
		{
			foreach (var mapper in _mappers)
			{
				try
				{
					var result = mapper.GetMember(columnName);
					if (result != null)
					{
						return result;
					}
				}
				catch (NotImplementedException)
				{
				}
			}
			return null;
		}
		
		
		public ConstructorInfo FindExplicitConstructor()
		{
			return _mappers
				.Select(mapper => mapper.FindExplicitConstructor())
				.FirstOrDefault(result => result != null);
		}
	}
  
}

@auroraegan your use case is supported out of the box by Dapper. Use this line in your application startup / entry point:

Dapper.DefaultTypeMap.MatchNamesWithUnderscores = true;

I have this code in my Global.asax.cs, is it possible to generalize this so that it runs against all classes? Or do I have to SqlMapper.SetTypeMap for every class that uses the column attribute?

protected void Application_Start()
{
    var mapper = (SqlMapper.ITypeMap)Activator
        .CreateInstance(typeof(ColumnAttributeTypeMapper<>)
        .MakeGenericType(typeof(User)));
    SqlMapper.SetTypeMap(typeof(User), mapper);
}

Follow-up, I found a solution that does exactly what I was looking for. https://stackoverflow.com/a/20969521/618186.
I will say that I don't know if it's the best approach because it's reflecting on the entire assembly, but since it's only being done once per application start it should be terrible. That's the trade-off of making the coding easier.

Here is the important part that you need. Add this to the ColumnAttributeTypeMapper.cs that @kalebpederson wrote.

public static class TypeMapper
{
	public static void Initialize(string @namespace)
	{
		var types = from assem in AppDomain.CurrentDomain.GetAssemblies().ToList()
				from type in assem.GetTypes()
				where type.IsClass && type.Namespace == @namespace
				select type;

		types.ToList().ForEach(type =>
		{
			var mapper = (SqlMapper.ITypeMap)Activator
				.CreateInstance(typeof(ColumnAttributeTypeMapper<>)
								.MakeGenericType(type));
			SqlMapper.SetTypeMap(type, mapper);
		});
	}
}

Put this in your Global.asax.cs

protected void Application_Start()
{
    TypeMapper.Initialize("YourNamespace.Entities");
}

I can use it to 'query',but can't use to 'update' or 'insert'.

I followed everything here, created all classes of ColumnAttributeTypeMapper.
Add on Startup =>
services.AddAutoMapper(System.Reflection.Assembly.GetExecutingAssembly());
TypeMapper.Initialize("Dapper.Core.Entities");
My Class
public class Product
{
[Column("Id_Prod")]
public int Id { get; set; }
}
Repository
public async Task<IReadOnlyList> GetAllAsync()
{
var sql = "SELECT * FROM Products";
using (var connection = new SqlConnection(configuration.GetConnectionString("DefaultConnection")))
{
connection.Open();
var result = await connection.QueryAsync(sql);
return result.ToList();
}
}
Dapper 1.50 and AutoMapper 8.0
I feel it's missing one little detail, but i'm searching for days.