Conversion from type ‘DBNull’ to type ‘String’ is not valid

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The quick and dirty fix:

hfSupEmail.Value = dt.Rows(0)("SupEmail").ToString()

or for C#:

hfsupEmail.Value = dt.Rows[0]["SupEmail"].ToString();

This works very well when your eventual target and the source data are already strings, because any extra .ToString() call for something that’s already a string is likely to be optimized into a no-op by the jitter, and if it’s NULL the resulting DBNull.Value.ToString() expression produces the empty string you want.

However, if you’re working with non-string types, you may end up doing significant extra work, especially with something like a DateTime or numeric value where you want specific formatting. Remember, internationalization concerns mean parsing and composing date and number values are actually surprisingly expensive operations; doing “extra” work to avoid those operations is often more than worth it.

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