SQL Server Express connection string for Entity Framework Code First

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The problem with your connection string here is:

<add name="TrempimModel"
     connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;
                       AttachDBFilename=|DataDirectory|aspnetdb.sdf;
                       User Instance=true"
     providerName="System.Data.SqlClient" />

You’re basically defining what “server” you’re connecting to – but you’re not saying what database inside the file to connect to. Also – the file extension for SQL Server Express database files is .mdf (not .sdf – that’s SQL Server Compact Edition) – you need to take that into account, too! (was a typo, according to comment by OP).

You need to define an extra database=.... (or Initial Catalog=.....) in your connection string:

<add name="TrempimModel"
     connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;
                       database=YourDatabaseName;
                       AttachDBFilename=|DataDirectory|aspnetdb.mdf;
                       User Instance=true"
     providerName="System.Data.SqlClient" />

Then it should work just fine.

For more background and tons of samples, check out the ConnectionStrings web site.

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