Isn't an Int64 equal to a long in C#?

Question

long is Int64 in .NET; it is just an alias in C#. Your problem is casting the return value to long and unless we know the type coming back from your query for sure, we would not know why you get an error. SQL BigInt must be convertable to long.

If it is the COUNT(*) which is coming back, then it is Int32. You need to use the Convert class:

long l = Convert.ToInt64(selectCommand.ExecuteScalar());

If you're thinking that your counts are going to overflow an int/Int32, you ought to use COUNT_BIG() in your SQL instead - it has the correct return type.

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As to why the casts aren't working, I'm not sure. The following C#:

System.Data.SqlClient.SqlCommand cmd = new System.Data.SqlClient.SqlCommand();
long lCount = (long)cmd.ExecuteScalar();
Int64 iCount = (Int64)cmd.ExecuteScalar();

Compiles to this IL:

L_0000: nop 
L_0001: newobj instance void [System.Data]System.Data.SqlClient.SqlCommand::.ctor()
L_0006: stloc.0 
L_0007: ldloc.0 
L_0008: callvirt instance object [System.Data]System.Data.Common.DbCommand::ExecuteScalar()
L_000d: unbox.any int64
L_0012: stloc.1 
L_0013: ldloc.0 
L_0014: callvirt instance object [System.Data]System.Data.Common.DbCommand::ExecuteScalar()
L_0019: unbox.any int64
L_001e: stloc.2 
L_001f: ret 

That is, they appear to compile to identical code.