Why Does My MSSQL Connection String Fail with 'The underlying provider failed on Open'?

Troubleshooting MSSQL Connection String Errors: "The underlying provider failed on Open"

Recently, attempting to modify a connection string to eliminate an .mdf file resulted in the error "The underlying provider failed on Open." While the connection string itself was valid, several underlying factors could cause this persistent problem.

One possibility, highlighted in a similar previous issue, is the use of Integrated Security. This requires the IIS user to have the correct database access permissions. If Entity Framework and transactions are used, the automatic connection management by Entity Framework might be the culprit. Each database call opens and closes a connection, potentially creating multiple connections within a single transaction, requiring MSDTC intervention.

To fix this, the code was modified to explicitly open the connection outside the transaction:

<code class="language-csharp">using (DatabaseEntities context = new DatabaseEntities())
{
    context.Connection.Open();
    // ... remaining code ...
}</code>

This isolated connection opening resolved the problem. This approach provides a valuable solution for debugging MSSQL connection issues.

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