gistfile1.cs

    public interface IMessage
    {
        T GetContent<T>() where T : class;
    }

    public abstract class MessageBase<TContentType> : IMessage where TContentType : class
    {
        TContentType Content { get; }

        public abstract TContentType GetContent<ContentType>();

        // I want to replace this with the GetContent above
        public T GetContent<T>() where T : class
        {
            throw new NotImplementedException();
        }
    }

public interface IMessage
{
    object GetContent();
}

public interface IMessage<T> : IMessage where T : class
{
    T GetContent();
}

public abstract class MessageBase<TContentType> : IMessage<TContentType> where TContentType : class
{
    TContentType Content { get; set; }

    public abstract TContentType GetContent();

    object IMessage.GetContent()
    {
        return GetContent();
    }
}

Another potential solution.

public interface IMessage
{
    object GetContent();
}

public interface ITypedMessage<TContentType> : IMessage where TContentType : class
{
    TContentType Content { get; }

    new TContentType GetContent();
}

public class WhisperMessage : ITypedMessage<string>
{
    public string Content
    {
        get
        {
            throw new NotImplementedException();
        }
    }

    public new string GetContent()
    {
        throw new NotImplementedException();
    }

    object IMessage.GetContent()
    {
        throw new NotImplementedException();
    }
}