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@kalebpederson
Last active November 25, 2023 01:47
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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);
}
}
}
@mishrsud
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@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;

@dannylloyd
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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);
}

@dannylloyd
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dannylloyd commented Aug 4, 2021

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");
}

@wuzhenda
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wuzhenda commented Jul 7, 2022

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

@RamseJones
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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.

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