How to Fix the 'The Underlying Provider Failed on Open' Error in Entity Framework When Connecting to MSSQL?

Troubleshooting the "Underlying Provider Failed on Open" Error in Entity Framework and MSSQL

The dreaded "The underlying provider failed on Open" error frequently arises when Entity Framework attempts to connect to an MSSQL database. The root cause is almost always an improperly configured connection string.

For connections to SQL databases without an .mdf file, ensure your connection string adheres to this structure:

<add connectionString="metadata=res://*/conString.csdl|res://*/conString.ssdl|res://*/conString.msl;provider=System.Data.SqlClient;provider connection string='Data Source=.\SQL2008;Initial Catalog=NData;Integrated Security=True;Connect Timeout=30;User Instance=True;MultipleActiveResultSets=True'" name="conString" providerName="System.Data.EntityClient" />

Crucial Connection String Parameters:

• Data Source: The name of your SQL Server instance (e.g., .SQL2008 for a local instance). Double-check this; a typo here is a common culprit.
• Initial Catalog: The name of the database you're targeting. Again, verify accuracy.
• Integrated Security: Setting this to True utilizes Windows Authentication.
• Connect Timeout: Specifies the connection attempt's maximum duration (in seconds). Increase this if your server is slow to respond.
• User Instance: Using True creates a user-specific database instance.
• MultipleActiveResultSets: True enables multiple result sets within a single connection.

Transaction-Related Issues:

Another potential source of this error involves Entity Framework transactions. Each database operation within a transaction automatically opens and closes a connection. This can create problems with MSDTC (Microsoft Distributed Transaction Coordinator).

Solution: Manual Connection Management

To circumvent MSDTC issues, explicitly open the connection before using it:

using (DatabaseEntities context = new DatabaseEntities())
{
    context.Connection.Open();
    // Your database operations here...
}

By carefully reviewing your connection string and employing manual connection management when necessary, you can effectively resolve the "Underlying Provider Failed on Open" error.

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