I can't input parameters for use ADODB to execute SQL procedure

HI,

SQL PROCEDURE
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CREATE PROCEDURE P_test @uid nvarchar(20) ,@ret nvarchar(1000) output AS
if(@uid='') set @uid=N'2222'
set @ret = N'select * from player where uid="'+@uid+N'"'
GO

C# CODES
--------------
...
com.CommandType = ADODB.CommandTypeEnum.adCmdStoredProc;
com.CommandText = "P_test";
string rets = "";
ADODB.Parameter pu = com.CreateParameter("@uid",ADODB.DataTypeEnum.adVarWChar,ADODB.ParameterDirectionEnum.adParamInput,20,"xxx");
com.Parameters.Append(pu);
ADODB.Parameter ret = conn.com.CreateParameter("@ret",ADODB.DataTypeEnum.adVarWChar,ADODB.ParameterDirectionEnum.adParamOutput,1000,rets);
com.Parameters.Append(pret);
object o = "",o1 = "";
com.Execute(out o,ref o1,ADODB.CommandTypeEnum.adCmdStoredProc);
rets = conn.com.Parameters["@ret"].Value.ToString();
...
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RESULTS rets = "select * from player where uid=\"2222\"";
------------
why not the value of 'xxx' don't transcation into the procedure

thks