无法使用实体框架读取从存储过程返回的值

我已经编写了一个存储过程，如果找到，将返回行ID，如果没有找到，将创建行并返回新创建行的ID。我在c#函数中检索这个返回值时遇到了问题。

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存储过程:

PROCEDURE [TagDatabase].[GetTagID] 
    -- Add the parameters for the stored procedure here
    @TagName as varchar(200), 
    @UserID int = 0,
    @TagID int = 0 OUTPUT
AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;
    select @TagID=ID from Tag where Name = @TagName and UserID=@UserID;
    IF NULLIF(@TagID, '') IS NULL
    BEGIN
        DECLARE @OutputTbl TABLE (ID INT)
        INSERT INTO Tag(Name, ParentID, UserID)
        OUTPUT INSERTED.ID INTO @OutputTbl(ID)
        VALUES (@TagName, 0, @UserID);
        select @TagID=ID from @OutputTbl

    END
    select @TagID
END

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我的函数来检索这个返回值:

ObjectParameter objParam = new ObjectParameter("TagID", typeof(long));
context.GetTagID("newTag", 10, objParam);
context.SaveChanges();
long id = Convert.ToInt64(Convert.ToInt64(id)));