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

LINQ to Entities Error: ToString() Method Not Supported

Migrating data between databases like MySQL and SQL Server can sometimes trigger this LINQ to Entities error:

"LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression."

This happens when you try to compare a string with a database field using the ToString() method within a LINQ query. The problem is that ToString() isn't a standard SQL function, and LINQ to Entities can't translate it into equivalent SQL code.

Solution: Using a Temporary Variable

The simplest solution is to store the string result of ToString() in a variable before using it in your LINQ query:

var strItem = item.Key.ToString();

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

This avoids the direct use of ToString() within the LINQ expression, resolving the translation issue.

Alternative: Employing SqlFunctions

In certain scenarios, the SqlFunctions helper class provides an alternative. It offers database-specific functions that LINQ to Entities can translate:

IQueryable<entity> pages = from p in context.pages
                           where p.Serial == SqlFunctions.StringConvert(item.Key)
                           select p;

SqlFunctions.StringConvert() performs a similar conversion to ToString() but in a way that's compatible with the database's SQL dialect.

Explanation:

The core issue lies in how LINQ to Entities processes expressions. It doesn't execute ToString() directly; instead, it attempts to translate the entire expression into SQL. Because there's no direct SQL equivalent for ToString(), the translation fails, leading to the exception. The solutions above circumvent this by either pre-processing the string or using a database-compatible function.

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