关于sql server：Dapper Execute返回-1值？

Dapper Execute returns -1 value?

以下SQL查询在给定值可用时返回1，在不可用时返回0，并且在SQL Server Management Studio中工作正常，

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select
case
when exists (select * from [dbo].[user]
where userName = 'admin'
and password = 'admin')
then cast(1 as bit)
else cast(0 as bit)
end

使用精简工具ORM的相同查询如下：

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public int login(string userName, string password)
{
string sql ="select case when exists(select * from user where userName = @usernamepara and password = @passwordpara) then cast(1 as bit) else cast(0 as bit )end";

using(IDbConnection conn = dbConnection)
{
conn.Open();

var res = conn.Execute(sql,
param: new { usernamepara = userName,
passwordpara = password });
conn.Close();
return res;
}
}

但是，当调用此方法时，对于匹配和不匹配的记录，它返回-1。怎么了