WithoutVoid.cs

public interface ICommandHandler<in TCommand, out TResult>
{
    TResult Execute(TCommand command);
}

public interface ICommandHandler<in TCommand> : ICommandHandler<TCommand, Unit> { }

public struct Unit
{
    // This is really a hack around the lack of a proper Void type...
    public static Unit Value { get; } = new Unit();
}

public class CommandHandler : ICommandHandler<string>
{
    public Unit Execute(string command)
    {
        return Unit.Value; // Have to return Unit here :(
    }
}

WithVoid.cs

public interface ICommandHandler<in TCommand, out TResult>
{
    TResult Execute(TCommand command);
}

public interface ICommandHandler<in TCommand> : ICommandHandler<TCommand, void>
{
}

public class CommandHandler : ICommandHandler<string>
{
    public void Execute(string command)
    {
        // No need to return anything! :D
    }
}