How can I create expression to be used in another expression?

I use Linq2Sql to access DB and Repository pattern implemented for that.

public abstract class RepositoryBase<T, TDb> : IRepository<T>
{
    public IQueryable<T> GetAll()
    {
        return GetTable().Select(GetConverter());
    }
}

public class ProductRepository
    : RepositoryBase<IProduct, DbData.Product>
{
    protected override Table<DbData.Product> GetTable()
    {
        return Context.CustomerProducts;
    }

    protected override Expression<Func<DbData.Product, IProduct>> GetConverter()
    {
        return dbEntity =>
             (ProdType.ProdTypeEnum)dbEntity.ProdId==ProdType.ProdTypeEnum.MyType
                    ? (new Product1{...}) as IProduct
                    : (new Product2{...}) as IProduct
    }
}

I need to modify mentioned code in this way:

    protected override Expression<Func<DbData.Product, IProduct>> GetConverter()
    {
        return dbEntity => 
             (ProdType.ProdTypeEnum)dbEntity.ProdId==ProdType.ProdTypeEnum.MyType
             || (ProdType.ProdTypeEnum)dbEntity.ProdId==ProdType.ProdTypeEnum.My2ndType
             || (ProdType.ProdTypeEnum)dbEntity.ProdId==ProdType.ProdTypeEnum.My3rdType
                    ? (new Product1{...}) as IProduct
                    : (new Product2{...}) as IProduct
    }

As I will have same check in other places I would like to separate a function:

public stati开发者_JS百科c bool IsProductOfType1(ProdType.ProdTypeEnum eProdTypeId)
{
    return eProdTypeIdd==ProdType.ProdTypeEnum.MyType
             || eProdTypeId==ProdType.ProdTypeEnum.My2ndType
             || eProdTypeId==ProdType.ProdTypeEnum.My3rdType
}


    protected override Expression<Func<DbData.Product, IProduct>> GetConverter()
    {
        return dbEntity => 
             IsProductOfType1((ProdType.ProdTypeEnum)dbEntity.ProdId)
                    ? (new Product1{...}) as IProduct
                    : (new Product2{...}) as IProduct
    }

The following case throws an exception:

Method 'Boolean IsProductOfType1(Int32)' has no supported translation to SQL

It seems like inside of 'GetConverter' method I need to use not a method, but an expression. But I don't know what is syntax of that.

Please advise. Thanks a lot!

The problem lies here:

public static bool IsProductOfType1(ProdType.ProdTypeEnum eProdTypeId)

This method should return an Expression<T> instead of bool. Any call to external methods won't be converter by LINQ to SQL