In various database tables I have both a property and a value column. I'm using Linq to SQL to access the database.
I'm writing a method which returns a dictionary containing the properties/values retrieved from the given database table:
private static Dictionary<string, string> GetProperties<T>(Table<T> table) { Dictionary<string, string> properties = new Dictionary<string, string>(); foreach (var row in table) { properties[row.Property]=row.Value; } return properties; }
Upon compiling, I get:
Error 1 The type 'T' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method 'System.Data.Linq.Table<TEntity>'
I've tried searching for this error message without luck.
Searching StackOverflow, this question seems similar, though regarding a parameter List: Generic List<T> as parameter on method - though the parameter still isn't a reference type in the answers to that question, either.
Reading the C# Programming Guide on MSDN: http://msdn.microsoft.com/en-us/library/twcad0zb(VS.80).aspx I see their examples all pass the parameters by reference. However, I can't see how to pass by reference in my particular case, since the generic type is just for specifying the generic type of Table.
Any pointers would be much appreciated.
PS: Appologies if it takes time for me to accept an answer, as this feature is currently not accessible (I'm blind and use a screen reader).
This happens because of how Table<T>
is declared:
public sealed class Table<TEntity> : IQueryable<TEntity>, IQueryProvider, IEnumerable<TEntity>, ITable, IQueryable, IEnumerable, IListSource where TEntity : class // <-- T must be a reference type!
The compiler is complaining because your method has no constraints on T
, which means that you could accept a T
which doesn't conform to the specification of Table<T>
.
Thus, your method needs to be at least as strict about what it accepts. Try this instead:
private static Dictionary<string, string> GetProperties<T>(Table<T> table) where T : class
Just add the constraint where T : class
to your method declaration.
This is required because Table<TEntity>
has a where TEntity : class
constraint. Otherwise your generic method could be called with a struct type parameter, which would require the CLR to instantiate Table<TEntity>
with that struct type parameter, which would violate the constraint on Table<TEntity>
.
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