SendModel_Request.cs

public enum Network { Bitcoin, Ethereum }

//This interface hides ethereum/bitcoin inconsistencies and provides unified interface to view model.
//This make sense only until ETH/BTC has very similar UI and workflow.
//This interface is private to SendViewModel since it is designed with it specific tasks in mind.
interface ISendRequest
{	
	decimal AvailableFunds { get; }
	string Recipient { get; set; }
	decimal Fee { get; set; }
	void ValidateRecipeint(Ivalidator validator);
	void Send();
}

//This view model holds all data required to send cryptocurrency to some recipient
public class SendViewModel : DataContext, IWeakListener
{
	//_sendRequest actually holds all data 	
	//View model delegates most properties to send request and 
	//User fills it by changing properties of this view model
	ISendRequest _sendRequest;

	public SendViewModel()
	{		
		SendCommand = new AsyncCommand(Send, _ => !_.IsRunning);
		PropertyChanged += ValidateCalculatedProperties;

		//JavvyContext holds all global services.

		//Subscribe to events which comes not from UI but from our model.
		//Subscriptions are implemented as weak references to avoid memory leaks.
		JavvyContext.MessageHub.Subscribe("Bitcoin.AvailableFunds", this);
		JavvyContext.MessageHub.Subscribe("Ethereum.AvailableFunds", this);
		//Init _sendRequest
		Network = Network.Bitcoin;
	}

	//Change AvailableFunds property when we read corresponding msesage from blockhain network
	void IWeakListener.OnMessage(Notification notification)
	{
		RaisePropertyChanged(nameof(AvailableFunds));
	}
	
	public decimal Amount 
	{ 
		get => this._sendRequest.Amount;
		set
		{
			this.Validator.CheckProperty(value).MoreOrEqThan(0);
			if (value != this._sendRequest.Amount)
			{
				this._sendRequest.Amount = value;
				RaisePropertyChanged(nameof(Amount));
				OnTotalTransactionAmountChanged();		
			}
		}
	}

	//Only valid for bitcoin	
	public decimal Fee 
	{ 
		get => this._sendRequest.Fee;
		set
		{
			//Fee cannot be less than 0
			this.Validator.CheckProperty(value).MoreOrEqThan(0).LessThan();
			if (value != this._sendRequest.Fee)
			{
				this._sendRequest.Fee = value;
				RaisePropertyChanged(nameof(Fee));
				OnTotalTransactionAmountChanged();
			}
		}
	}

	public decimal TotalTransactionAmount
	{
		get 
		{
			return Amount + Fee;
		}		
	}

	string _recipient;
	public string Recipient 
	{ 
		get => this._sendRequest.Recipient;
		set
		{
			var checker = this.Validator.CheckProperty(value).NotEmpty();
			this._sendRequest.ValidateRecipient(Validator);			
			if (value != this._sendRequest.Recipient)
			{
				this._sendRequest.Recipient = value;
				RaisePropertyChanged(nameof(Recipient));
			}
		}
	}

	Network _network;
	public Network Network 
	{ 
		get => _network;
		set
		{
			if (value != _network)
			{
				_network = value;			
				var request = _network == Network.Bitcoin ? new BitcoinSendRequest() : EthereumSendRequest();
				//Copy data from model to request to preserve it over Netwrok property changes
				//Mapper is an instance of AutoMapper configured to map between SendViewModel and ISendRequest
				JavvyContext.Mapper.Map(this, request);
				this._sendRequest = request;

				RaisePropertyChanged(nameof(Network));
				RaisePropertyChanged(nameof(Fee));
				OnAvailableFundsChanged();
				Validate();
			}
		}
	}

	public decimal AvailableFunds 
	{
		get 
		{
			return this._sendRequest.AvailableFunds;
		}
	}

	private void OnTotalTransactionAmountChanged()
	{
		ValidateTotalAmount();
		RaisePropertyChanged(nameof(TotalTransactionAmount));
	}

	private void ValidateTotalAmount()
	{
		this.Validator.Check(TotalTransactionAmount, nameof(TotalTransactionAmount))
		 		 .LessThan(AvailableFunds);
	}

	private void OnAvailableFundsChanged()
	{
		RaisePropertyChanged(nameof(AvailableFunds));
		//TotalTransactionAmount depends on AvailableFunds;		
		OnTotalTransactionAmountChanged();
	}

	private async Task Send(object arg, CancellationToken token)
	{
		_sendStrategy.Send(Recipient, Amount, Fee);
	}
}

abstract class BaseSendRequest : ISendRequest
{
	protected readonly TransactionDto _dto = new TrdansactionDto();
	public string Recipient { get => _dto.Recipient; set => _dto.Recipient = value; }
	public decimal Amount { get => _dto.Amount; set => _dto.Amount = value; }

	public abstract void ValidateRecipient(IValidator validator);

	public void Send()
	{
		JavvyContext.TransactionService.Send(_dto);
	}
}

class BitcoinSendRequest : BaseSendRequest
{
	public decimal Fee { get => _dto.Fee; set => _dto.Fee = value; }

	public decimal AvailableFunds => JavvyContext.User.Bitcoin.AvailableFunds;

	public Error ValidateRecipient(Ivalidator validator)
	{
		this.validator.Check(nameof(Recipient), recipient).NotEmpty().Convert(BitcoinAddress.Parse).NotNull();
	}
}

class EthereumSendRequest : BaseSendRequest
{	
	public decimal Fee { get => 0; set => {} }

	public decimal AvailableFunds => JavvyContext.User.Ethereum.AvailableFunds;

	public void ValidateRecipient(Check<string> recipient)
	{
		this.validator.Check(nameof(Recipient), recipient).NotEmpty().Convert(EthereumAddress.Parse).NotNull();
	}
}

public interface IWeakListener
{
	void OnMessage(Notification notification);
}