How to Resolve 'LINQ to Entities does not recognize the method 'Double Parse(System.String)''?

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

Issue

When running a LINQ query that attempts to apply Double.Parse to string properties in a database context, users may encounter the following error:

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

The Problem

Entity Framework (EF) is designed to translate LINQ queries into SQL commands that can be executed on the database. However, the Double.Parse method is not a valid SQL function. Therefore, EF cannot translate it into SQL and execute the query successfully.

Solution

To resolve this issue, you need to create a custom method that performs the same operation as Double.Parse and make EF aware of it so that it can translate it into SQL.

Step 1: Define the Custom Function

In the Entity Data Model (EDMX) file associated with your data context, add the following function definition within the element:

<Function Name="ParseDouble" ReturnType="Edm.Double">
  <Parameter Name="stringvalue" Type="Edm.String" />
  <DefiningExpression>
    cast(stringvalue as Edm.Double)
  </DefiningExpression>
</Function>

This function tells EF how to convert a string value to a double value in SQL.

Step 2: Implement the Custom Method

In the partial class that defines your data context, create a method matching the signature of the function you defined in the EDMX file:

public partial class MyDataContext
{
    [EdmFunction("YourModel", "ParseDouble")]
    public static double ParseDouble(string stringvalue)
    {
        return Double.Parse(stringvalue);
    }
}

Step 3: Use the Custom Function in LINQ Query

Now you can use the ParseDouble function in your LINQ queries:

var query = context.MyEntities
    .Select(e => new
    {
        DoubleValue = ParseDouble(e.StringValue)
    });

By following these steps, you can enable EF to translate your LINQ queries that use Double.Parse into valid SQL, resolving the "LINQ to Entities does not recognize the method" error.

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