Can continue to build a partially buit transaction

NBitcoin.Tests/transaction_tests.cs

@@ -174,15 +174,15 @@ public void CanBuildAnyoneCanPayTransaction()
 				.Send(carlaKey.PubKey.ID, "0.01")
 				//and Carla pays Alice
 				.Send(aliceKey.PubKey.ID, "0.02")
-				.CoverOnly("0.02")
+				.CoverOnly("0.01")
 				.SetChange(aliceBobRedeemScript.ID)
 				// Bob does not sign anything yet
 				.BuildTransaction(false);
 
 			Assert.True(unsigned.Outputs.Count == 3);
 			Assert.True(unsigned.Outputs[0].IsTo(aliceBobRedeemScript.ID));
-			//Only 0.02 should be covered, not 0.03 so 0.48 goes back to Alice+Bob
-			Assert.True(unsigned.Outputs[0].Value == Money.Parse("0.48"));
+			//Only 0.01 should be covered, not 0.03 so 0.49 goes back to Alice+Bob
+			Assert.True(unsigned.Outputs[0].Value == Money.Parse("0.49"));
 
 
 			Assert.True(unsigned.Outputs[1].IsTo(carlaKey.PubKey.ID));
@@ -198,17 +198,20 @@ public void CanBuildAnyoneCanPayTransaction()
 					.AddKeys(aliceKey)
 					.SignTransaction(unsigned, SigHash.All | SigHash.AnyoneCanPay);
 
-			var carlaCoins = GetCoinSource(carlaKey, "1.0", "0.8", "0.6", "0.2", "0.04");
+			var carlaCoins = GetCoinSource(carlaKey, "1.0", "0.8", "0.6", "0.2", "0.05");
+
+			//Scenario 1 : Carla knows aliceBobCoins so she can calculate how much coin she need to complete the transaction
 			//Carla fills and signs
 			txBuilder = new TransactionBuilder();
 			var carlaSigned = txBuilder
 				.AddCoins(aliceBobCoins)
 				.Then()
 				.AddKeys(carlaKey)
-				//Carla should complete 0.01, but with 0.03 of fees, she should have a coins of 0.04
+				//Carla should complete 0.02, but with 0.03 of fees, she should have a coins of 0.05
 				.AddCoins(carlaCoins)
+				.ContinueToBuild(aliceSigned)
 				.SendFees("0.03")
-				.CompleteTransaction(aliceSigned)
+				.CoverTheRest()
 				.BuildTransaction(true);
 
 
@@ -221,6 +224,31 @@ public void CanBuildAnyoneCanPayTransaction()
 
 			txBuilder.AddCoins(carlaCoins);
 			Assert.True(txBuilder.Verify(bobSigned));
+
+
+			//Scenario 2 : Carla is told by Bob to complete 0.05 BTC
+			//Carla fills and signs
+			txBuilder = new TransactionBuilder();
+			carlaSigned = txBuilder
+				.AddCoins(aliceBobCoins)
+				.Then()
+				.AddKeys(carlaKey)
+				.AddCoins(carlaCoins)
+				//Carla should complete 0.02, but with 0.03 of fees, she should have a coins of 0.05
+				.ContinueToBuild(aliceSigned)
+				.CoverOnly("0.05")
+				.BuildTransaction(true);
+
+
+			//Bob review and signs
+			txBuilder = new TransactionBuilder();
+			bobSigned = txBuilder
+				.AddCoins(aliceBobCoins)
+				.AddKeys(bobKey)
+				.SignTransaction(carlaSigned);
+
+			txBuilder.AddCoins(carlaCoins);
+			Assert.True(txBuilder.Verify(bobSigned));
 		}
 
 		private ICoin[] GetCoinSource(Key destination, params Money[] amounts)

NBitcoin/TransactionBuilder.cs

@@ -945,32 +945,45 @@ public TransactionBuilder CoverOnly(Money amount)
 		Transaction _CompletedTransaction;
 
 		/// <summary>
-		/// Find Coins to complete TxOut of the transaction (Does not support colored coins)
+		/// Allows to keep building on the top of a partially built transaction
 		/// </summary>
-		/// <param name="transaction"></param>
+		/// <param name="transaction">Transaction to complete</param>
 		/// <returns></returns>
-		public TransactionBuilder CompleteTransaction(Transaction transaction)
+		public TransactionBuilder ContinueToBuild(Transaction transaction)
 		{
 			if(_CompletedTransaction != null)
 				throw new InvalidOperationException("Transaction to complete already set");
 			_CompletedTransaction = transaction.Clone();
+			return this;
+		}
 
-			var spent = transaction.Inputs.Select(txin =>
-				{
-					var c = FindCoin(txin.PrevOut);
-					if(c == null)
-						throw CoinNotFound(txin);
-					if(c is IColoredCoin)
-						return null;
-					return c;
-				})
-				.Where(c => c != null)
-				.Select(c => c.Amount)
-				.Sum();
-
-			var toComplete = transaction.TotalOut - spent;
+		/// <summary>
+		/// Will cover the remaining amount of TxOut of a partially built transaction (to call after ContinueToBuild)
+		/// </summary>
+		/// <returns></returns>
+		public TransactionBuilder CoverTheRest()
+		{
+			if(_CompletedTransaction == null)
+				throw new InvalidOperationException("A partially built transaction should be specified by calling ContinueToBuild");
+
+			var spent = _CompletedTransaction.Inputs.Select(txin =>
+			{
+				var c = FindCoin(txin.PrevOut);
+				if(c == null)
+					throw CoinNotFound(txin);
+				if(c is IColoredCoin)
+					return null;
+				return c;
+			})
+					.Where(c => c != null)
+					.Select(c => c.Amount)
+					.Sum();
+
+			var toComplete = _CompletedTransaction.TotalOut - spent;
 			CurrentGroup.Builders.Add(ctx =>
 			{
+				if(toComplete < Money.Zero)
+					return Money.Zero;
 				return toComplete;
 			});
 			return this;