Dapper getting “Specified cast is not valid.” for ReturnValue parameter value

问题

I am trying to use SCOPE_IDENTITY to return a long primary key back to c# using the ReturnValue option for DynamicParameter.

Here is sample code from the Dapper website:

var p = new DynamicParameters();
p.Add("@a", 11);
p.Add("@b", dbType: DbType.Int32, direction: ParameterDirection.Output);
p.Add("@c", dbType: DbType.Int32, direction: ParameterDirection.ReturnValue);

cnn.Execute("spMagicProc", p, commandType: commandType.StoredProcedure); 

int b = p.Get<int>("@b");
int c = p.Get<int>("@c");

Instead of returning int, I would prefer to do the following as my primary key field should be bigint

var p = new DynamicParameters();
p.Add("@a", 11);
p.Add("@b", dbType: DbType.Int32, direction: ParameterDirection.Output);
p.Add("@c", dbType: DbType.Int64, direction: ParameterDirection.ReturnValue);

cnn.Execute("spMagicProc", p, commandType: commandType.StoredProcedure); 

int b = p.Get<int>("@b");
int c = p.Get<long>("@c");

In my proc, I am using "RETURN SCOPE_IDENTITY()".

However doing this seems to result in a "Specified cast is not valid." exception.

回答1:

The return value from a stored procedure is always implicitly an integer, i.e. int. As such, you can only treat it as an integer. If you try to unbox it as a long, it will fail.

Options:

if the value will fit, just treat it as int in the .NET side
otherwise use an out parameter of type bigint, and treat it as long on the .NET side
or use select and Query<long>(...).Single()

回答2:

If i remember correctly, SCOPE_IDENTITY() returns a decimal (http://msdn.microsoft.com/en-us/library/ms190315.aspx), hence the invalid cast exception.

You need to cast it to bigint (in the SQL) for it to work as you want it.

来源：https://stackoverflow.com/questions/17197018/dapper-getting-specified-cast-is-not-valid-for-returnvalue-parameter-value

标签

stored-procedures

dapper