如何将用户定义的类型传递给SqlParameterCollection.AddWithValue? [英] How should I pass a user-defined type to SqlParameterCollection.AddWithValue?

问题描述

我有一个名为StudentID的自定义数据类型，该类型隐式转换为字符串.

I have a custom data type called StudentID, which has an implicit conversion to string.

当我将StudentID实例传递给SqlCommand.Parameters.AddWithValue(作为值)并执行命令时，我收到以下错误:

When I pass a StudentID instance to SqlCommand.Parameters.AddWithValue (as the value) and execute the command, I receive the following error:

No mapping exists from object type StudentID to a known managed provider native type.

为像SqlDbType.NVarChar这样的参数指定类型无济于事.唯一有效的方法是将StudentID值显式转换为字符串，这违背了进行隐式强制转换的目的.

Specifying a type for the parameter like SqlDbType.NVarChar doesn't help. The only thing that works is to explicitly cast the StudentID value to a string, which defeats the purpose of having the implicit cast.

我猜该框架没有考虑可用的转换.我可以对StudentID类做些什么使命令处理它，而无需显式转换为原始类型?

I guess the framework does not consider available conversions. Is there anything I can do to the StudentID class to make the command handle it without requiring an explicit cast to a primitive type?

这篇MSDN文章讨论了如何处理原始类型，但不是用户定义的类型.最接近的匹配将是object，它说它是作为Variant传递的.

This MSDN article talks about how primitive types are handled, but not user-defined types. The closest match would be object, which it says is passed as a Variant.

确切的故障点在签名为private static MetaType GetMetaTypeFromValue(Type dataType, object value, bool inferLen)的System.Data.SqlClient.MetaType.GetMetaTypeFromValue方法中.反射表明它正在寻找原始类型，如果TypeCode是对象，它将寻找Sql *数据类型，例如SqlBoolean，因此它似乎正在寻找非常特殊的原始类型或Sql *类型.

The exact point of failure is in the System.Data.SqlClient.MetaType.GetMetaTypeFromValue method, with signature private static MetaType GetMetaTypeFromValue(Type dataType, object value, bool inferLen). Reflection reveals that it looks for primative types, and if the TypeCode is object, it looks for Sql* data types such as SqlBoolean, so it seems to be looking for very specific kinds of primitive or Sql* types.

推荐答案

据我所知，您不能传递UDT.仅允许使用基本类型或Sql *类型.

As I know you can't pass UDTs. Only primitive or Sql* types are allowed.

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