为什么只有使用单独的字段时，SqlDataReader才能使用DataTable？

Question

We have a quiet large SQL Server database (about 30 million rows in the biggest tables). When I try to run a stored procedure on it I have a strange problem. It is working if I use this code:

cmd = new SqlCommand("query", conn);    
cmd.CommandType = CommandType.StoredProcedure;    
cmd.CommandText = "StoredProcedureName";    
cmd.Parameters.Add("myParameter", myParameter);    
cmd.CommandTimeout = 600;

DataTable table = new DataTable();    
var restmp = cmd.ExecuteReader();    
table.Load(restmp);

But it is running forever if I use this one:

cmd = new SqlCommand("query", conn);    
cmd.CommandType = CommandType.StoredProcedure;    
cmd.CommandText = "StoredProcedureName";    
cmd.Parameters.Add("myParameter", myParameter);    
cmd.CommandTimeout = 600;

DataTable table = new DataTable();    
table.Load(cmd.ExecuteReader());      

What can cause this?

Answer 1

You may be playing var roulette. The var declaration determines the type of variable automatically. And when you embed cmd.ExecuteReader() within table.load, you're avoiding that declaration process. Instead, try explicitly casting cmd.ExecuteReader() with SqlDataReader via table.load on your second code snippet.

Link to C# var reference

"Beginning in Visual C# 3.0, variables that are declared at method scope can have an implicit type var. An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type..."