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How do I define a generic constraint so I can use the coalescence operator

I am trying to define a generic class

public abstract class RepositoryBase<TDatabase, TKey, T> : IRepository<TKey, T> 
    where T : class
    where TDatabase : IDatabase
{
    private TDatabase db;

    private readonly IDbSet<T> dbset;

    protected IDatabaseFactory<TDatabase> DatabaseFactory { get; private set; }

    protected TDatabase Database
    {
        get
        {
            return db ?? (db = DatabaseFactory.Get());
        }
    }
    ...
}

On line, the return db ?? (db = DatabaseFactory.Get());

compiler complains about "Left operand" ?? "operator must have a reference or null type"

I understand the error, but I don't know how to set a constraint on the TDatabase type parameter so that the compiler knows it is a reference or null type.

How to make the compiler happy?

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4 answers


You must specify what TDatabase

is the reference type

where TDatabase : class, IDatabase

MSDN, Restrictions on Type Parameters (C # Programming Guide)



where T: class The type argument must be a reference type; this also applies to any class, interface, delegation, or array type.

MSDN, ?? Operator (C # reference) :

the operator is called the null coalescing operator and is used to define a default value for null value types or reference types. This returns the left-hand operand if the operand is non-zero; otherwise, it returns the right-hand operand.

+7


source 


where TDatabase : class, IDatabase


+1


source


Can you change your inclusion limit class

?

public abstract class RepositoryBase<TDatabase, TKey, T> : IRepository<TKey, T>  
    where T : class 
    where TDatabase : class, IDatabase 
{
    //... 
}
+1


source


try it

public abstract class RepositoryBase<TDatabase, TKey, T> : IRepository<TKey, T>
    where T : class
    where TDatabase : class, IDatabase
{
}
+1


source






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