Question

I’m not sure if/how I can check that the constructor actually exists before calling activator in this code (untested so might have bugs but hopefully the intention is clear).

Its as if I wanted a template constraint that says “where T has a constructor with signature S”.

public class EntityContainerFactory
{
    public EntityContainerFactory(string sqlServerName, string databaseName, string metaData)
        : this(sqlServerName, databaseName, metaData, "System.Data.EntityClient")
    {
    }

    public EntityContainerFactory(string sqlServerName, string databaseName, string metaData, string dataProviderName)
    {
        SqlServerName = sqlServerName;
        DatabaseName = databaseName;
        Metadata = metaData;
        DataProviderName = dataProviderName;
    }

    // --> IS THERE ANY WAY TO CHECK THAT T HAS 
    //     A CONSTRUCTOR THAT TAKES AN ENTITY CONNECTION?
    public T Create<T>()
    {
        return (T)Activator.CreateInstance(typeof(T), CreateEntityConnection());
    }

    EntityConnection CreateEntityConnection()
    {
        SqlConnectionStringBuilder sqlBuilder = new SqlConnectionStringBuilder();
        sqlBuilder.DataSource = SqlServerName;
        sqlBuilder.InitialCatalog = DatabaseName;
        sqlBuilder.IntegratedSecurity = true;

        EntityConnectionStringBuilder entityBuilder = new EntityConnectionStringBuilder();
        entityBuilder.Provider = DataProviderName;
        entityBuilder.ProviderConnectionString = sqlBuilder.ToString();
        entityBuilder.Metadata = Metadata;

        return new EntityConnection(entityBuilder.ConnectionString);
    }

    public string DatabaseName { get; set; }

    public string SqlServerName { get; set; }

    public string DataProviderName { get; set; }

    private string metaData;
    public string Metadata
    {
        get
        {
            string result;
            if (!this.metaData.StartsWith("res://"))
            {
                result = string.Format(@"res://*/{0}.csdl|res://*/{0}.ssdl|res://*/{0}.msl", this.metaData);
            }
            else
            {
                result = this.metaData;
            }
            return result;
        }
        set
        {
            this.metaData = value;
        }
    }
}

Answer 1

Have you considered using interfaces instead of factories? You can get much better results, and you don’t run into problems like this.

Anyway:

where T : new()

Only works for parameterless constructors. When you have that constraint, you can do the following, without reflection:

T obj = new T();

You’re going to have to check manually.

ConstructorInfo ctor = typeof(T).GetConstructor(new Type[] { EntityConnection });
if (ctor == null)
{
    // Handle an unwanted type.
}
else
{
    return (T)ctor.Invoke(entityConnection);
}