Why Does LINQ to Entities Throw a 'Method 'System.String ToString()' Not Recognized' Error, and How Can I Fix It?

Troubleshooting "Method 'System.String ToString()' Not Recognized" in LINQ to Entities

Database migration projects often encounter the frustrating "LINQ to Entities does not recognize the method 'System.String ToString()' method" error. This occurs because LINQ to Entities can't translate the ToString() method call within your LINQ query into a database-compatible expression. The problem specifically arises when using ToString() on a string field directly within a LINQ query.

The Solution: Pre-process String Conversion

The simplest fix is to avoid using ToString() inside the LINQ query itself. Instead, perform the string conversion beforehand, assigning the result to a variable. This separates the string manipulation from the database query translation. Example:

<code class="language-csharp">string strItem = item.Key.ToString();

IQueryable<entity> pages = from p in context.pages
                           where p.Serial == strItem
                           select p;</code>

By pre-converting the string, ToString() executes before the LINQ query is translated to SQL, preventing the error.

Alternative: Entity Framework's SqlFunctions (Later Versions)

Newer Entity Framework versions offer a more elegant solution via the SqlFunctions helper class. This class provides a ToString() method specifically designed for use within LINQ queries, eliminating the need for temporary variables. This allows for more compact and readable code.

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