This error "Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints" can be caused by different reasons:
1. truly violating non-null, unique, or foreign-key constraints
2. a field length in the db doesn't match the one in the dataset. This can easily happen when you use Visual Studio designer.

Normally we would start looking at the first one. If we cannot find any problem, then we need to look at the second one - check all the MaxLength property of the columns in your DataTable. This can be an easy fix in many cases.

Alternatively you can set EnforceConstraints = False. However, it is not recommended because you will lose the guard for the table adapters.

If you are still tricked by this famous error message, then you need to find out exactly which field in the dataset schema is causing the problem by checking the errors on each row in a table in the typed dataset. It can be done quickly in a watch window in Visual Studio by adding a watch on <NameOfDataSetWithErrors>.<NameOfDataTableWithErrors>.GetErrors()

See this link for more details.