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POC# LINQ to SQL: Refactoring this Generic GetByID method
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I wrote the following method. <pre><code>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); } </code></pre> Basically it's a method in a Generic class where <code>T</code> is a class in a DataContext. The method gets the table from the type of T (<code>GetTable</code>) 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 <code>GetType</code> on the property, but this is not very convenient because all the elements of the table have to be enumerated and converted to a <code>List</code>. How can I refactor this method to avoid a <code>ToList</code> on the whole table? [Update] The reason I can't execute the <code>Where</code> directly on the table is because I receive this exception: <blockquote> Method 'System.Reflection.PropertyInfo[] GetProperties()' has no supported translation to SQL. </blockquote> Because <code>GetProperties</code> can't be translated to SQL. [Update] Some people have suggested using an interface for T, but the problem is that the <code>T</code> 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 <a href="https://stackoverflow.com/questions/735140/c-linq-to-sql-refectoring-this-generic-getbyid-method/735209#735209">Neil Williams</a>' suggestion, but I'm still having problems. Here are excerpts of the code: Interface: <pre><code>public interface IHasID { int ID { get; set; } } </code></pre> DataContext [View Code]: <pre><code>namespace MusicRepo_DataContext { partial class Artist : IHasID { public int ID { get { return ArtistID; } set { throw new System.NotImplementedException(); } } } } </code></pre> Generic Method: <pre><code>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)); } } </code></pre> The exception is being thrown on this line: <code>return table.SingleOrDefault(e => e.ID.Equals(id));</code> and the exception is: <blockquote> <code>System.NotSupportedException: The member 'MusicRepo_DataContext.IHasID.ID' has no supported translation to SQL.</code> </blockquote> [Update] Solution: With the help of <a href="https://stackoverflow.com/questions/735140/c-linq-to-sql-refectoring-this-generic-getbyid-method/735888#735888">Denis Troller</a>'s posted answer and the link to the post at the <a href="http://mikehadlow.blogspot.com/2008/03/using-irepository-pattern-with-linq-to.html" rel="nofollow noreferrer">Code Rant blog</a>, I finally managed to find a solution: <pre><code>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(); } </code></pre>
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C# LINQ to SQL: Refactoring this Generic GetByID method
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