How to Check byte array empty or not?

You must swap the order of your test:

From:

if (Attachment.Length > 0 && Attachment != null)

To:

if (Attachment != null && Attachment.Length > 0 )

The first version attempts to dereference Attachment first and therefore throws if it's null. The second version will check for nullness first and only go on to check the length if it's not null (due to "boolean short-circuiting").

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[EDIT] I come from the future to tell you that with later versions of C# you can use a "null conditional operator" to simplify the code above to:

if (Attachment?.Length > 0)
        

.Net V 4.6 OR C # 6.0

Try This

 if (Attachment?.Length > 0)

Just do

if (Attachment != null  && Attachment.Length > 0)

From && Operator

The conditional-AND operator (&&) performs a logical-AND of its bool operands, but only evaluates its second operand if necessary.