By default the DbContext is created only with a single, parameterless constructor. As the exception message says you will have to either remove the connection string from the data source (you can just go in the designer.cs file, find the connection string and delete the whole line) or to provide a second constructor. For the latter to work you will have to create a constructor with a string argument and pass that string to the base constructor. For more information please check the first note in this
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