Why is my stored procedure not setting the value?

Question

When executing a stored procedure through C#, the value stored in the table does not change.

But when the stored procedure is executed through SQL Server Management Studio, it is executed correctly.

This is my C# code:

DatabaseLibrary.ParametersClear();

DatabaseLibrary.command.Parameters.Add(new SqlParameter("@guid", SqlDbType.UniqueIdentifier)).Value = formGuid;
DatabaseLibrary.command.Parameters.Add(new SqlParameter("@val", SqlDbType.Float)).Value = (Sumsrc.Net * Convert.ToDouble(ParityTxt.Text));
DatabaseLibrary.command.Parameters.Add(new SqlParameter("@in", SqlDbType.Bit)).Value = input ? 1 : 0;
DatabaseLibrary.command.Parameters.Add(new SqlParameter("@operation", SqlDbType.Bit)).Value = 1;

DatabaseLibrary.ExecuteStoredProcedure(StoredProcedure.AccountUpdate);

Stored procedure code:

CREATE PROCEDURE [dbo].[AccountUpdate]
    @guid uniqueidentifier,
    @val float,
    @in bit, -- 1 or 0
    @operation bit  -- 1 or 0
AS
BEGIN
    DECLARE @oldVal float

    IF (@in > 0)
    BEGIN
        IF (@operation > 0)
        BEGIN
            SET @oldVal = (SELECT ISNULL(Debit, 0) FROM ac00 WHERE GUID = @guid)

            UPDATE ac00  
            SET [Debit] = (@oldVal + @val) 
            WHERE GUID = @guid
        END
        ELSE
        BEGIN
            SET @oldVal = (SELECT ISNULL(Debit, 0) FROM ac00 WHERE guid = @guid)

            UPDATE ac00  
            SET [Debit] = (@oldVal - @val) 
            WHERE GUID = @guid
        END
    END
    ELSE
    BEGIN
        IF (@operation > 0)
        BEGIN
            SET @oldVal = (SELECT ISNULL(Credit, 0) FROM ac00 WHERE GUID = @guid)

            UPDATE ac00  
            SET [Credit] = (@oldVal + @val) 
            WHERE GUID = @guid
        END
        ELSE
        BEGIN
            SET @oldVal = (SELECT ISNULL(Credit, 0) FROM ac00 WHERE GUID = @guid)

            UPDATE ac00  
            SET [Credit] = (@oldVal - @val) 
            WHERE GUID = @guid
        END
    END
END