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Check for NULL values

=IIf(Fields!YourColumn.Value, Fields!YourColumn.Value, "Unknown")
=IIf(Fields!YourColumn.Value > 0, Fields!YourColumn.Value, "Unknown")

1 2	=IIf(Fields!YourColumn.Value, Fields!YourColumn.Value, "Unknown") =IIf(Fields!YourColumn.Value > 0, Fields!YourColumn.Value, "Unknown")

Check string contains substring

=IIf(Fields!Notes.Value.IndexOf("sample") >= 0,"Black","White")
=IIf(Fields!Notes.Value.ToLowerInvariant().Contains("sample"),"Black","White")

1 2	=IIf(Fields!Notes.Value.IndexOf("sample") >= 0,"Black","White") =IIf(Fields!Notes.Value.ToLowerInvariant().Contains("sample"),"Black","White")