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;
}

Answer 2

What if you rework this to use GetTable().Where(...), and put your filtering there?

That would be more efficient, since the Where extension method should take care of your filtering better than fetching the entire table into a list.

Answer 3

Some thoughts...

Just remove the ToList() call, SingleOrDefault works with an IEnumerably which I presume table is.

Cache the call to e.GetType().GetProperties().First() to get the PropertyInfo returned.

Cant you just add a constraint to T that would force them to implement an interface that exposes the Id property?

Answer 4

Ok, check this demo implementation. Is attempt to get generic GetById with datacontext(Linq To Sql). Also compatible with multi key property.

using System;
using System.Data.Linq;
using System.Data.Linq.Mapping;
using System.Linq;
using System.Reflection;
using System.Collections.Generic;

public static class Programm
{
    public const string ConnectionString = @"Data Source=localhost\SQLEXPRESS;Initial Catalog=TestDb2;Persist Security Info=True;integrated Security=True";

    static void Main()
    {
        using (var dc = new DataContextDom(ConnectionString))
        {
            if (dc.DatabaseExists())
                dc.DeleteDatabase();
            dc.CreateDatabase();
            dc.GetTable<DataHelperDb1>().InsertOnSubmit(new DataHelperDb1() { Name = "DataHelperDb1Desc1", Id = 1 });
            dc.GetTable<DataHelperDb2>().InsertOnSubmit(new DataHelperDb2() { Name = "DataHelperDb2Desc1", Key1 = "A", Key2 = "1" });
            dc.SubmitChanges();

            Console.WriteLine("Name:" + GetByID(dc.GetTable<DataHelperDb1>(), 1).Name);
            Console.WriteLine("");
            Console.WriteLine("");
            Console.WriteLine("Name:" + GetByID(dc.GetTable<DataHelperDb2>(), new PkClass { Key1 = "A", Key2 = "1" }).Name);
        }
    }

    //Datacontext definition
    [Database(Name = "TestDb2")]
    public class DataContextDom : DataContext
    {
        public DataContextDom(string connStr) : base(connStr) { }
        public Table<DataHelperDb1> DataHelperDb1;
        public Table<DataHelperDb2> DataHelperD2;
    }

    [Table(Name = "DataHelperDb1")]
    public class DataHelperDb1 : Entity<DataHelperDb1, int>
    {
        [Column(IsPrimaryKey = true)]
        public int Id { get; set; }
        [Column]
        public string Name { get; set; }
    }

    public class PkClass
    {
        public string Key1 { get; set; }
        public string Key2 { get; set; }
    }
    [Table(Name = "DataHelperDb2")]
    public class DataHelperDb2 : Entity<DataHelperDb2, PkClass>
    {
        [Column(IsPrimaryKey = true)]
        public string Key1 { get; set; }
        [Column(IsPrimaryKey = true)]
        public string Key2 { get; set; }
        [Column]
        public string Name { get; set; }
    }

    public class Entity<TEntity, TKey> where TEntity : new()
    {
        public static TEntity SearchObjInstance(TKey key)
        {
            var res = new TEntity();
            var targhetPropertyInfos = GetPrimaryKey<TEntity>().ToList();
            if (targhetPropertyInfos.Count == 1)
            {
                targhetPropertyInfos.First().SetValue(res, key, null);
            }
            else if (targhetPropertyInfos.Count > 1) 
            {
                var sourcePropertyInfos = key.GetType().GetProperties(BindingFlags.Instance | BindingFlags.Public);
                foreach (var sourcePi in sourcePropertyInfos)
                {
                    var destinationPi = targhetPropertyInfos.FirstOrDefault(x => x.Name == sourcePi.Name);
                    if (destinationPi == null || sourcePi.PropertyType != destinationPi.PropertyType)
                        continue;

                    object value = sourcePi.GetValue(key, null);
                    destinationPi.SetValue(res, value, null);
                }
            }
            return res;
        }
    }

    public static IEnumerable<PropertyInfo> GetPrimaryKey<T>()
    {
        foreach (var info in typeof(T).GetProperties().ToList())
        {
            if (info.GetCustomAttributes(false)
            .Where(x => x.GetType() == typeof(ColumnAttribute))
            .Where(x => ((ColumnAttribute)x).IsPrimaryKey)
            .Any())
                yield return info;
        }
    }
    //Move in repository pattern
    public static TEntity GetByID<TEntity, TKey>(Table<TEntity> source, TKey id) where TEntity : Entity<TEntity, TKey>, new()
    {
        var searchObj = Entity<TEntity, TKey>.SearchObjInstance(id);
        Console.WriteLine(source.Where(e => e.Equals(searchObj)).ToString());
        return source.Single(e => e.Equals(searchObj));
    }
}

Result:

SELECT [t0].[Id], [t0].[Name]
FROM [DataHelperDb1] AS [t0]
WHERE [t0].[Id] = @p0

Name:DataHelperDb1Desc1


SELECT [t0].[Key1], [t0].[Key2], [t0].[Name]
FROM [DataHelperDb2] AS [t0]
WHERE ([t0].[Key1] = @p0) AND ([t0].[Key2] = @p1)

Name:DataHelperDb2Desc1

Answer 5

Maybe executing a query might be a good idea.

public static T GetByID(int id)
    {
        Type type = typeof(T);
        //get table name
        var att = type.GetCustomAttributes(typeof(TableAttribute), false).FirstOrDefault();
        string tablename = att == null ? "" : ((TableAttribute)att).Name;
        //make a query
        if (string.IsNullOrEmpty(tablename))
            return null;
        else
        {
            string query = string.Format("Select * from {0} where {1} = {2}", new object[] { tablename, "ID", id });

            //and execute
            return dbcontext.ExecuteQuery<T>(query).FirstOrDefault();
        }
    }

Answer 6

Regarding:

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

The simple workaround to your initial problem is to specify an Expression. See below, it works like a charm for me.

public interface IHasID
{
    int ID { get; set; }
}
DataContext [View Code]:

namespace MusicRepo_DataContext
{
    partial class Artist : IHasID
    {
        [Column(Name = "ArtistID", Expression = "ArtistID")]
        public int ID
        {
            get { return ArtistID; }
            set { throw new System.NotImplementedException(); }
        }
    }
}