Why Does LINQ to Entities Throw a 'Method Not Recognized' Exception When Using ToString()?

LINQ to Entities: Addressing the "Method Not Recognized" Exception

Database migration often leads to method recognition problems. A common error is:

"LINQ to Entities doesn't recognize the method 'System.String ToString()' method, and this method can't be translated into a store expression."

This typically happens when comparing a string property with a string expression, like this:

<code>IQueryable<entity> pages =
    from p in context.pages
    where p.Serial == item.Key.ToString()
    select p;</code>

Understanding the Root Cause:

The ToString() method isn't directly executed but treated as a MethodGroup. LINQ attempts to translate this group into an equivalent SQL expression. If the target database lacks a direct ToString() equivalent, the translation fails.

Resolution: Pre-Storing the String Value:

The simplest fix is to store the string result in a variable before the comparison:

<code>string strItem = item.Key.ToString();

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

This removes the ToString() call from the query, preventing the translation error.

Alternative: Leveraging SqlFunctions:

The SqlFunctions helper class offers database-specific functions for LINQ to Entities queries. For ToString(), you can try:

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

Note the cast to (decimal?) which is often necessary depending on the underlying database type of p.Serial. SqlFunctions can streamline code and avoid temporary variables, but requires careful consideration of data types.

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