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Trace Dapper.NET Source Code - TypeHandler custom Mapping logic & its underlying logic

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Github Link : Trace-Dapper.NET-Source-Code

18. TypeHandler custom Mapping logic & its underlying logic

When you want to customize some attribute Mapping logic, you can use TypeHandler in Dapper

  1. Create a class to inherit SqlMapper.TypeHandler
  2. Assign the class to be customized to the generic, e.g: JsonTypeHandler<custom_type>: SqlMapper.TypeHandler<custom_type>
  3. Override Parse method to custom Query logic, and override SetValue method to custom Create,Delete,Updte logic
  4. If there are multiple class Parse and SetValue share the same logic, you can change the implementation class to a generic method. The custom class can be specified in AddTypeHandler, which can avoid creating a lot of class, eg: JsonTypeHandler<T>: SqlMapper.TypeHandler< T> where T: class

Example : when User level is changed, the change action will be automatically recorded in the Log field.

public class JsonTypeHandler<T> : SqlMapper.TypeHandler<T> 
  where T : class
  public override T Parse(object value)
    return JsonConvert.DeserializeObject<T>((string)value);

  public override void SetValue(IDbDataParameter parameter, T value)
    parameter.Value = JsonConvert.SerializeObject(value);

public void Main()
  SqlMapper.AddTypeHandler(new JsonTypeHandler<List<Log>>()); 

  using (var ts = new TransactionScope())
  using (var cn = new SqlConnection(@"Data Source=(localdb)\MSSQLLocalDB;Integrated Security=SSPI;Initial Catalog=master;"))

    cn.Execute("create table [User] (Name nvarchar(200),Age int,Level int,Logs nvarchar(max))");

    var user = new User()
      Name = "Wei",
      Age = 26,
      Level = 1,
      Logs = new List<Log>() {
        new Log(){Time=DateTime.Now,Remark="CreateUser"}

    // add
      cn.Execute("insert into [User] (Name,Age,Level,Logs) values (@Name,@Age,@Level,@Logs);", user);

      var result = cn.Query("select * from [User]");

    // Level up
      user.Level = 9;
      user.Logs.Add(new Log() {Remark="UpdateLevel"});
      cn.Execute("update [User] set Level = @Level,Logs = @Logs where Name = @Name", user);
      var result = cn.Query("select * from [User]");



public class User
  public string Name { get; set; }
  public int Age { get; set; }
  public int Level { get; set; }
  public List<Log> Logs { get; set; }

public class Log
  public DateTime Time { get; set; } = DateTime.Now;
  public string Remark { get; set; }
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Then trace the TypeHandler source code logic, which needs to be traced in two parts: SetValue, Parse

The underlying logic of SetValue

  1. AddTypeHandlerImpl method to manage the addition of cache

  2. When creating a dynamic AddParameter method in the CreateParamInfoGenerator method Emit, if there is data in the TypeHandler cache of the Mapping type, Emit adds an action to call the SetValue method.

if (handler != null)
  il.Emit(OpCodes.Call, typeof(TypeHandlerCache<>).MakeGenericType(prop.PropertyType).GetMethod(nameof(TypeHandlerCache<int>.SetValue))); // stack is now [parameters] [[parameters]] [parameter]
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  1. LookupDbType will be used when calling the AddParameters method at Runtime to determine whether there is a custom TypeHandler



  1. Then pass the created Parameter to the custom TypeHandler.SetValue method


Finally, the C# code converted from IL

    public static void TestMeThod(IDbCommand P_0, object P_1)
        User user = (User)P_1;
        IDataParameterCollection parameters = P_0.Parameters;
        IDbDataParameter dbDataParameter3 = P_0.CreateParameter();
        dbDataParameter3.ParameterName = "Logs";
        dbDataParameter3.Direction = ParameterDirection.Input;
        SqlMapper.TypeHandlerCache<List<Log>>.SetValue(dbDataParameter3, ((object)user.Logs) ?? ((object)DBNull.Value));
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It can be found that the generated Emit IL will get our implemented TypeHandler from TypeHandlerCache, and then call the implemented SetValue method to run the set logic, and TypeHandlerCache uses generic type to save different handlers in Singleton mode according to different generics. This has the following advantages :

  1. Same handler can be obtained to avoid repeated creation of objects
  2. Because it is a generic type, reflection actions can be avoided when the handler is taken, and efficiency can be improved


Parse corresponds to the underlying principle

The main logic is when the GenerateDeserializerFromMap method Emit establishes the dynamic Mapping method, if it is judged that the TypeHandler cache has data, the Parse method replaces the original Set attribute action.


View the IL code generated by the dynamic Mapping method:

IL_0000: ldc.i4.0   
IL_0001: stloc.0    
IL_0002: newobj     Void .ctor()/Demo.User
IL_0007: stloc.1    
IL_0008: ldloc.1    
IL_0009: dup        
IL_000a: ldc.i4.0   
IL_000b: stloc.0    
IL_000c: ldarg.0    
IL_000d: ldc.i4.0   
IL_000e: callvirt   System.Object get_Item(Int32)/System.Data.IDataRecord
IL_0013: dup        
IL_0014: stloc.2    
IL_0015: dup        
IL_0016: isinst     System.DBNull
IL_001b: brtrue.s   IL_0029
IL_001d: unbox.any  System.String
IL_0022: callvirt   Void set_Name(System.String)/Demo.User
IL_0027: br.s       IL_002b
IL_0029: pop        
IL_002a: pop        
IL_002b: dup        
IL_002c: ldc.i4.1   
IL_002d: stloc.0    
IL_002e: ldarg.0    
IL_002f: ldc.i4.1   
IL_0030: callvirt   System.Object get_Item(Int32)/System.Data.IDataRecord
IL_0035: dup        
IL_0036: stloc.2    
IL_0037: dup        
IL_0038: isinst     System.DBNull
IL_003d: brtrue.s   IL_004b
IL_003f: unbox.any  System.Int32
IL_0044: callvirt   Void set_Age(Int32)/Demo.User
IL_0049: br.s       IL_004d
IL_004b: pop        
IL_004c: pop        
IL_004d: dup        
IL_004e: ldc.i4.2   
IL_004f: stloc.0    
IL_0050: ldarg.0    
IL_0051: ldc.i4.2   
IL_0052: callvirt   System.Object get_Item(Int32)/System.Data.IDataRecord
IL_0057: dup        
IL_0058: stloc.2    
IL_0059: dup        
IL_005a: isinst     System.DBNull
IL_005f: brtrue.s   IL_006d
IL_0061: unbox.any  System.Int32
IL_0066: callvirt   Void set_Level(Int32)/Demo.User
IL_006b: br.s       IL_006f
IL_006d: pop        
IL_006e: pop        
IL_006f: dup        
IL_0070: ldc.i4.3   
IL_0071: stloc.0    
IL_0072: ldarg.0    
IL_0073: ldc.i4.3   
IL_0074: callvirt   System.Object get_Item(Int32)/System.Data.IDataRecord
IL_0079: dup        
IL_007a: stloc.2    
IL_007b: dup        
IL_007c: isinst     System.DBNull
IL_0081: brtrue.s   IL_008f
IL_0083: call       System.Collections.Generic.List`1[Demo.Log] Parse(System.Object)/Dapper.SqlMapper+TypeHandlerCache`1[System.Collections.Generic.List`1[Demo.Log]]
IL_0088: callvirt   Void set_Logs(System.Collections.Generic.List`1[Demo.Log])/Demo.User
IL_008d: br.s       IL_0091
IL_008f: pop        
IL_0090: pop        
IL_0091: stloc.1    
IL_0092: leave      IL_00a4
IL_0097: ldloc.0    
IL_0098: ldarg.0    
IL_0099: ldloc.2    
IL_009a: call       Void ThrowDataException(System.Exception, Int32, System.Data.IDataReader, System.Object)/Dapper.SqlMapper
IL_009f: leave      IL_00a4
IL_00a4: ldloc.1    
IL_00a5: ret        
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Convert it into C# code to verify:

 public static User TestMeThod(IDataReader P_0)
    int index = 0;
    User user = new User();
    object value = default(object);
      User user2 = user;
      index = 0;
      object obj = value = P_0[0];
      index = 3;
      object obj4 = value = P_0[3];
      if (!(obj4 is DBNull))
        user2.Logs = SqlMapper.TypeHandlerCache<List<Log>>.Parse(obj4);
      user = user2;
      return user;
    catch (Exception ex)
      SqlMapper.ThrowDataException(ex, index, P_0, value);
      return user;
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