your code seems OK but what if both conditions not match? I have added default value for final condition.
=Switch(
IsNumeric(Fields!C2.Value) and Fields!C2.Value > 0, Fields!SeqNo.Value,
1=1, Fields!C2.Value
)
UPDATE:
for below:
if(C2 ==" ")
{
C2.Value = empty
}
else
{
C2.Value = Fields!SeqNo.Value
}
try
=Switch(
Fields!C2.Value = " ", "",
1=1, Fields!C2.Value
)