Dapper查询抛错误 Object must implement IConvertible.

原因

Dapper根据查询语句中的列名集合，缓存映射实体方法。
也就是说，如果查询的列数量、名称等相同，不管是不是同一个表、实体，所有查询映射为同一个实体，如果查询结果T不为缓存的实体，在转换时就会报错。

解决方案

查询字段（名词、数量）和实体保持唯一

错误信息

System.InvalidCastException : Object must implement IConvertible.
错误信息

 消息: 
    System.InvalidCastException : Object must implement IConvertible.
  堆栈跟踪: 
    Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
    SqlMapper.QueryImpl[T](IDbConnection cnn, CommandDefinition command, Type effectiveType)+MoveNext() 行 1105
    List`1.ctor(IEnumerable`1 collection)
    Enumerable.ToList[TSource](IEnumerable`1 source)
    SqlMapper.Query[T](IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Boolean buffered, Nullable`1 commandTimeout, Nullable`1 commandType) 行 721
    SqlMapperExtensions.GetPageAll[T](IDbConnection connection, Int32 page, Int32 perPageSize, Expression`1 sortPredicate, ISpecification`1 specification, Boolean isAsc, IDbTransaction transaction, Nullable`1 commandTimeout) 行 340
    DapperRepositoryContext.GetPage[TEntity,TKey](Int32 page, Int32 perPageSize, Expression`1 sortPredicate, Boolean isAsc, ISpecification`1 specification) 行 259
    RepositoryBase`2.GetPage(Int32 page, Int32 perPageSize, Expression`1 sortPredicate, Boolean isAsc, ISpecification`1 specification) 行 287
    RepositoryBaseTests.GetPageTest() 行 617

代码示例

public class TableAction
{
     public string Guid { get; set; }

     public string Code { get; set; }    

     public string Name { get; set; }   

 }
 public class TableObject
 {
     public string Guid { get; set; }

     public string Code { get; set; }

     public string Name { get; set; } 

     public string Description { get; set; }
 }

using (var conn = new Npgsql.NpgsqlConnection("User ID=postgres;Password=Bio!@#456;Host=192.168.1.182;Port=5432;Database=bup_dips;Pooling=true;"))
{
	conn.Open();
	var sql_action = "select guid , code ,name from bup_t_action limit 1;";
	var sql_object = "select guid , code ,name from bup_t_action_object limit 1;";
	//下面这句能执行成功
	var tableAction = await conn.QueryFirstAsync<TableAction>(sql_action);
	
	//这句执行失败
	var tableObject = await conn.QueryFirstAsync<TableObject>(sql_object);
	
}	

原因

执行查询

执行查询前会缓存中获取转换方法，如果没有在创建，后面会看到缓存Key为查询字段，暂时只知道读取缓存就好

var identity = new Identity(command.CommandText, command.CommandType, cnn, effectiveType, param?.GetType());

var info = GetCacheInfo(identity, param, command.AddToCache);

转换方法

序列化方法的实例是个结构体

 private struct DeserializerState
 {
      public readonly int Hash;
      public readonly Func<IDataReader, object> Func;

      public DeserializerState(int hash, Func<IDataReader, object> func)
      {
          Hash = hash;
          Func = func;
      }
  }

Key

Value

总结

while (reader.Read())
{
    object val = func(reader);
    //reader相同返回列，object val始终为最先执行的class：假设为A(class)
    //如果T为A ,则 val == null || val is T 为 true，能够强转成功
    if (val == null || val is T)
    {
        yield return (T)val;
    }
    //后执行的查询，假设对象T不是A，就会在这里报错
    //Convert.ChangeType转换失败
    //将A强转为T(类B、类C、类D)抛出错误
    //System.InvalidCastException : Object must implement IConvertible.
    else
    {
        yield return (T)Convert.ChangeType(val, convertToType, CultureInfo.InvariantCulture);
    }
}