Why Does LINQ to Entities Throw a 'System.String ToString()' Exception, and How Can I Fix It?

LINQ to Entities: Troubleshooting the "System.String ToString()" Error

When working with LINQ to Entities (LINQ2E) to query and manipulate data in a relational database, you might encounter the 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 a LINQ expression uses the ToString() method on a string in a way that can't be directly converted into a SQL equivalent.

Let's examine this scenario:

<code class="language-C#">using (var context = new Context())
{
    // ...

    foreach (var item in collection)
    {
        IQueryable<entity> pages = from p in context.pages
                                   where p.Serial == item.Key.ToString()
                                   select p;
        foreach (var page in pages)
        {
            DataManager.AddPageToDocument(page, item.Value);
        }
    }

    Console.WriteLine("Done!");
    Console.Read();
}</code>

Here, ToString() is called on item.Key (a string) within the where clause. LINQ2E struggles to translate this into SQL because ToString() lacks a direct SQL counterpart.

Solutions to the Problem

The most straightforward solution is to pre-assign the string value to a variable:

<code class="language-C#">var strItem = item.Key.ToString();

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

This works because LINQ2E now sees strItem as a constant, eliminating the need for translation.

An Alternate Solution (EF Core 2.1 and later):

EF Core 2.1 and later versions offer the SqlFunctions helper class, providing a StringConvert method specifically for LINQ2E expressions:

<code class="language-C#">IQueryable<entity> pages = from p in context.pages
                           where p.Serial == SqlFunctions.StringConvert((double?)item.Key) // Note the cast to (double?)
                           select p;</code>

Using either method resolves the "System.String ToString()" exception and ensures your LINQ2E queries execute correctly. Remember to handle potential null values appropriately, as shown by the (double?) cast in the SqlFunctions example. This is often necessary for database compatibility.

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