How to Integrate Custom SQL Functions with LINQ to Entities?

LINQ to Entities and Custom SQL Functions

LINQ to Entities allows developers to query data using a familiar syntax that closely resembles SQL. However, certain operations may require custom functions that cannot be directly translated into SQL. In such cases, developers must define a custom function in the Entity Data Model (EDMX) file and implement a matching method in code.

Defining a Custom Function in EDMX

Within the tag in the EDMX file, define a Function element representing your custom function. This element defines the translated Entity-SQL code, such as:

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

Implementing the Custom Function in Code

In the partial class generated from the EDMX file, implement a method that matches the custom function defined in the EDMX:

using System.Data.Objects.DataClasses;

public partial class YourObjectContext
{
    /// <summary>
    ///     This method exists for use in LINQ queries,
    ///     as a stub that will be converted to a SQL CAST statement.
    /// </summary>
    [EdmFunction("YourModel", "ParseDouble")]
    public static double ParseDouble(string stringvalue)
    {
        return Double.Parse(stringvalue);
    }
}

Applying the Custom Function in LINQ

In your LINQ statement, replace the corresponding function call with the implemented method:

model.Referring = Math.Round(_newSurveyResult.Select(m => string.IsNullOrEmpty(m.Question1) ? 0 : YourObjectContext.ParseDouble(m.Question1)).Average());

Custom Round Function

If the LINQ statement also includes a call to Math.Round, define a custom function in the EDMX:

<Function Name="Round" ReturnType="Edm.Double"> 
    <Parameter Name="input" Type="Edm.Double" /> 
    <DefiningExpression> 
        Round(input)
    </DefiningExpression> 
</Function> 

Additional Considerations

It is important to consult the EDM Canonical Functions list to ensure that the desired function can be translated into SQL. If a function cannot be translated, a different approach, such as implementing a stored procedure, may be necessary.

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