C# LINQ to SQL: Refactoring this Generic GetByID method

Question

I wrote the following method.

public T GetByID(int id)
{
    var dbcontext = DB;
    var table = dbcontext.GetTable<T>();
    return table.ToList().SingleOrDefault(e => Convert.ToInt16(e.GetType().GetProperties().First().GetValue(e, null)) == id);
}

Basically it's a method in a Generic class where T is a class in a DataContext.

The method gets the table from the type of T (GetTable) and checks for the first property (always being the ID) to the inputted parameter.

The problem with this is I had to convert the table of elements to a list first to execute a GetType on the property, but this is not very convenient because all the elements of the table have to be enumerated and converted to a List.

How can I refactor this method to avoid a ToList on the whole table?

[Update]

The reason I can't execute the Where directly on the table is because I receive this exception:

Method 'System.Reflection.PropertyInfo[] GetProperties()' has no supported translation to SQL.

Because GetProperties can't be translated to SQL.

[Update]

Some people have suggested using an interface for T, but the problem is that the T parameter will be a class that is auto generated in [DataContextName].designer.cs, and thus I cannot make it implement an interface (and it's not feasible implementing the interfaces for all these "database classes" of LINQ; and also, the file will be regenerated once I add new tables to the DataContext, thus loosing all the written data).

So, there has to be a better way to do this...

[Update]

I have now implemented my code like Neil Williams' suggestion, but I'm still having problems. Here are excerpts of the code:

Interface:

public interface IHasID
{
    int ID { get; set; }
}

DataContext [View Code]:

namespace MusicRepo_DataContext
{
    partial class Artist : IHasID
    {
        public int ID
        {
            get { return ArtistID; }
            set { throw new System.NotImplementedException(); }
        }
    }
}

Generic Method:

public class DBAccess<T> where T :  class, IHasID,new()
{
    public T GetByID(int id)
    {
        var dbcontext = DB;
        var table = dbcontext.GetTable<T>();

        return table.SingleOrDefault(e => e.ID.Equals(id));
    }
}

The exception is being thrown on this line: return table.SingleOrDefault(e => e.ID.Equals(id)); and the exception is:

System.NotSupportedException: The member 'MusicRepo_DataContext.IHasID.ID' has no supported translation to SQL.

[Update] Solution:

With the help of Denis Troller's posted answer and the link to the post at the Code Rant blog, I finally managed to find a solution:

public static PropertyInfo GetPrimaryKey(this Type entityType)
{
    foreach (PropertyInfo property in entityType.GetProperties())
    {
        ColumnAttribute[] attributes = (ColumnAttribute[])property.GetCustomAttributes(typeof(ColumnAttribute), true);
        if (attributes.Length == 1)
        {
            ColumnAttribute columnAttribute = attributes[0];
            if (columnAttribute.IsPrimaryKey)
            {
                if (property.PropertyType != typeof(int))
                {
                    throw new ApplicationException(string.Format("Primary key, '{0}', of type '{1}' is not int",
                                property.Name, entityType));
                }
                return property;
            }
        }
    }
    throw new ApplicationException(string.Format("No primary key defined for type {0}", entityType.Name));
}

public T GetByID(int id)
{
    var dbcontext = DB;

    var itemParameter = Expression.Parameter(typeof (T), "item");
    var whereExpression = Expression.Lambda<Func<T, bool>>
        (
        Expression.Equal(
            Expression.Property(
                 itemParameter,
                 typeof (T).GetPrimaryKey().Name
                 ),
            Expression.Constant(id)
            ),
        new[] {itemParameter}
        );
    return dbcontext.GetTable<T>().Where(whereExpression).Single();
}

Answer 1

What you need is to build an expression tree that LINQ to SQL can understand. Assuming your "id" property is always named "id":

public virtual T GetById<T>(short id)
{
    var itemParameter = Expression.Parameter(typeof(T), "item");
    var whereExpression = Expression.Lambda<Func<T, bool>>
        (
        Expression.Equal(
            Expression.Property(
                itemParameter,
                "id"
                ),
            Expression.Constant(id)
            ),
        new[] { itemParameter }
        );
    var table = DB.GetTable<T>();
    return table.Where(whereExpression).Single();
}

This should do the trick. It was shamelessly borrowed from this blog. This is basically what LINQ to SQL does when you write a query like

var Q = from t in Context.GetTable<T)()
        where t.id == id
        select t;

You just do the work for LTS because the compiler cannot create that for you, since nothing can enforce that T has an "id" property, and you cannot map an arbitrary "id" property from an interface to the database.

==== UPDATE ====

OK, here's a simple implementation for finding the primary key name, assuming there is only one (not a composite primary key), and assuming all is well type-wise (that is, your primary key is compatible with the "short" type you use in the GetById function):

public virtual T GetById<T>(short id)
{
    var itemParameter = Expression.Parameter(typeof(T), "item");
    var whereExpression = Expression.Lambda<Func<T, bool>>
        (
        Expression.Equal(
            Expression.Property(
                itemParameter,
                GetPrimaryKeyName<T>()
                ),
            Expression.Constant(id)
            ),
        new[] { itemParameter }
        );
    var table = DB.GetTable<T>();
    return table.Where(whereExpression).Single();
}


public string GetPrimaryKeyName<T>()
{
    var type = Mapping.GetMetaType(typeof(T));

    var PK = (from m in type.DataMembers
              where m.IsPrimaryKey
              select m).Single();
    return PK.Name;
}