Validate query string and value based on if query string is present

Scenario:

Validate query string for null or empty if only it exists

Solution:

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 public bool validate()
        {
            if (queryStringCollection.AllKeys.Any(k => k.StartsWith("field_name")))
            {
                fieldValue = queryStringCollection.Get("field_name");
                if (string.IsNullOrWhiteSpace(fieldValue) )
                {
                    throw customeError;
                }
            }
        }