标量值 function 来自 SQL 从 C# 调用时服务器不返回值

Question

I was given this function

CREATE FUNCTION [dbo].[GET_WEBGIS_ISSUE_NUM]
    ()
RETURNS VARCHAR(50)
AS 
BEGIN
    DECLARE @v_new_num int, @v_new_issue_num varchar(50);
    
    SET @v_new_num = (SELECT COUNT(*) + 1 
                      FROM [dbo].[WEBGIS_ISSUE] 
                      WHERE [ISSUE_NUM] LIKE  CONCAT(FORMAT(GETDATE(), 'yyMM'), '%'));

    IF @v_new_num < 10 
        SET @v_new_issue_num = CONCAT(FORMAT(GETDATE(), 'yyMM'), '00', @v_new_num);

    ELSE IF @v_new_num < 100
        SET @v_new_issue_num = CONCAT(FORMAT(GETDATE(), 'yyMM'), '00', @v_new_num);

    ELSE
        SET @v_new_issue_num = CONCAT(FORMAT(GETDATE(), 'yyMM'), @v_new_num);
    
    RETURN @v_new_issue_num 
END;

I tried calling it from the following C# code

SqlConnection cnn = new SqlConnection(connectionString);

SqlCommand cmd = new SqlCommand();
cmd.Connection = cnn;
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "[NEPS].[dbo].[GET_WEBGIS_ISSUE_NUM]";

//add any parameters the stored procedure might require
if (cmd.Connection.State == ConnectionState.Closed) //cmd.Connection.Open();
{
    cnn.Open();
    var o = cmd.ExecuteScalar();
    //blabla
    cnn.Close();
}

but when I debug the code, I kept on receiving null.

Notes: the connection is ok, it is connected, when I tried changing the function's name it yields an error and when I checked through the SQL Server it also returns an appropriate return value.

Answer 1

You're treating a scalar function as a Store Procedure, which is the wrong type for this type of execution. You need to 'CommandType.Text' with scalar functions.

Other notes on the C# part:

• use using blocks with SqlConnection and SqlCommand (let the using clause handles the dispose and close connection parts for you).
• the query should be declared as const string
• always end the query with a semicolon (even if it's running in the SQL Server without it).
• avoid using short names, choose a readable naming for your variables.

here is the C# code:

const string query = "SELECT [NEPS].[dbo].[GET_WEBGIS_ISSUE_NUM]();";

using(SqlConnection connection = new SqlConnection(connectionString))
    using(SqlCommand command = new SqlCommand(query, connection))
    {
        connection.Open();
        var result = command.ExecuteScalar();

        // do stuff
    }

For the function GET_WEBGIS_ISSUE_NUM perhaps you can avoid the extra IF s with this line:

CREATE FUNCTION [dbo].[GET_WEBGIS_ISSUE_NUM]
    ()
RETURNS VARCHAR(50)
AS 
BEGIN
    DECLARE @v_new_num int, @v_new_issue_num varchar(50);
    
    SET @v_new_num = (SELECT COUNT(*) + 1 
                      FROM [dbo].[WEBGIS_ISSUE] 
                      WHERE [ISSUE_NUM] LIKE  CONCAT(FORMAT(GETDATE(), 'yyMM'), '%'));

    SET @v_new_issue_num, FORMAT(GETDATE(), 'yyMM') + RIGHT('000' + CAST(@v_new_num AS VARCHAR), 4);
    
    RETURN @v_new_issue_num 
END;

Answer 2

It is more common to use a SELECT statement to return a scalar function result. When you use EXECUTE (due to CommandType.StoredProcedure ), you need to also specify a return parameter and retrieve the result from the parameter after execution:

var result = cmd.Parameters.Add("@result", SqlDbType.VarChar, 50);
result.Direction = ParameterDirection.ReturnValue;
cmd.ExecuteNonQuery(); //ExecuteScalar will work too but the result is null and you still need to use the parameter
var o = result.Value;

As noted in the comments to your question, consider the concurrency implications of this approach. Duplicate values will be returned until the row count changes.