How to Retrieve a Primary Key from a Stored Procedure's Output Parameter in C#?

Using stored procedure output parameters in C#

When retrieving data from a stored procedure, you often need to access output parameters to obtain additional information. In this example, a C# program encounters problems while trying to retrieve the primary key returned by the usp_InsertContract stored procedure.

The stored procedure is slightly modified to use SCOPE_IDENTITY(), as shown below:

CREATE PROCEDURE usp_InsertContract
(
    @ContractNumber varchar(7),
    @NewId int OUTPUT
)
AS
BEGIN
    INSERT INTO [dbo].[Contracts] (ContractNumber)
    VALUES (@ContractNumber)

    SELECT @NewId = SCOPE_IDENTITY()
END

The C# code has also been adjusted to correctly retrieve the output value:

// 打开连接并执行存储过程
conn.Open();
cmd.ExecuteNonQuery();

// 从@NewId读取输出值
int contractID = Convert.ToInt32(cmd.Parameters["@NewId"].Value);
conn.Close();

This modification should solve the problem by ensuring that the primary key is correctly returned as an output parameter and accessible in the C# program.

Note: Be sure to verify that the Contracts table has columns with the IDENTITY attribute configured correctly. Missing IDENTITY columns may also make it difficult to obtain the primary key value.

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