A Little Chat About Code Sensible Code Generation

At work we all try and do our best to be productive. And as software developers we all probably have to author classes, that follow a familiar pattern. So if like me you are looking to work smart, you will undoubtedly look into code that writes code.

This is good in my opinion, code generation can save a LOT of time. But the cardinal rule of clever code generation is never ever having to touch the generated code once it has been generated. If you do have to make changes by hand to a generated file, its all wrong and the entire process can no longer be trusted.

Now this perplexed me for a while as I had a very real demand to perform some calculations when a property changes. Now this would be cool and could possibly be made part of the generated code if you know about it in advance and have the correct knowledge to instruct your code generator about this in advance. We neither knew about these requirements up front or were able to build enough intelligence into our code generator to deal with this sort of this.

So you can imagine that our business analyst came along and said "when property X changes, use property Y and Z to work out Q". Mmmmm.

This would require me to do something when a property changes, this would mean manually changing my auto generated code. Oh no.

So I had a think about this, and luckily C# being the lovely language that it is has the answers. Partial classes / methods. All we need to do is provide the stubs in the generated code portion and provide hooks that the manually created partial class can use.

Here is an example.

The code generated part

  1:  using System;
  2:  using System.Collections.Generic;
  3:  using System.Linq;
  4:  using System.Text;
  5:
  6:  namespace NiceCodeGenStylee
  7:  {
  8:      public partial class Class1
  9:      {
 10:
 11:          private Int32 number1 = 0;
 12:          private Int32 number2 = 0;
 13:
 14:          /// <summary>
 15:          /// When this method in called the hand cranked
 16:          /// code should have the implementation
 17:          /// </summary>
 18:          partial void Number1PropertyChanging(
                           IntReportChanges args);
 19:
 20:          /// <summary>
 21:          /// When this method in called the hand cranked
 22:          /// code should have the implementation
 23:          /// </summary>
 24:          partial void Number1CodePropertyChanged(
                           Int32ItemChanges args);
 25:
 26:
 27:          /// <summary>
 28:          /// When this method in called the hand cranked
 29:          /// code should have the implementation
 30:          /// </summary>
 31:          partial void Number2PropertyChanging(
                           IntReportChanges args);
 32:
 33:          /// <summary>
 34:          /// When this method in called the hand cranked
 35:          /// code should have the implementation
 36:          /// </summary>
 37:          partial void Number2CodePropertyChanged(
                           Int32ItemChanges args);
 38:
 39:
 40:          /// <summary>
 41:          /// Number1 property
 42:          /// </summary>
 43:          public Int32 Number1
 44:          {
 45:              get { return number1; }
 46:              set
 47:              {
 48:                  //is it different
 49:                  if(value != number1)
 50:                  {
 51:                      Int32 oldValue = number1;
 52:
 53:                      //what are the changes
 54:                      Int32ItemChanges possibleValueChanges
 55:                          = new Int32ItemChanges(
                                     oldValue, value);
 56:
 57:                      //report the possible change, allows non generated file to
 58:                      //respond to the change, or cancel it
 59:                      IntReportChanges changes =
 60:                          new IntReportChanges
 61:                          {
 62:                              Values = possibleValueChanges
 63:                          };
 64:                      Number1PropertyChanging(changes);
 65:
 66:                      //if there was no cancellation
 67:                      if (!changes.Cancel)
 68:                      {
 69:                          number1 = value;
 70:                          //report the change, allows non generated file to
 71:                          //respond to the change
 72:                          Number1CodePropertyChanged(
                                  possibleValueChanges);
 73:                      }
 74:                  }
 75:
 76:              }
 77:          }
 78:
 79:          /// <summary>
 80:          /// Number2 property
 81:          /// </summary>
 82:          public Int32 Number2
 83:          {
 84:              get { return number2; }
 85:              set
 86:              {
 87:                  //is it different
 88:                  if (value != number2)
 89:                  {
 90:                      Int32 oldValue = number2;
 91:
 92:                      //what are the changes
 93:                      Int32ItemChanges possibleValueChanges
 94:                          = new Int32ItemChanges(oldValue, value);
 95:
 96:                      //report the possible change, allows non generated file to
 97:                      //respond to the change, or cancel it
 98:                      IntReportChanges changes =
 99:                          new IntReportChanges
100:                          {
101:                              Values = possibleValueChanges
102:                          };
103:                      Number1PropertyChanging(changes);
104:
105:                      //if there was no cancellation
106:                      if (!changes.Cancel)
107:                      {
108:                          number2 = value;
109:                          //report the change, allows non generated file to
110:                          //respond to the change
111:                          Number1CodePropertyChanged(
                                  possibleValueChanges);
112:                      }
113:                  }
114:
115:              }
116:          }
117:
118:
119:      }
120:  }

Where the manually created code part, can use these partial method stubs to do calculations when a property changes or even cancel a changing property based on some condition.

 1:  using System;
 2:  using System.Collections.Generic;
 3:  using System.Linq;
 4:  using System.Text;
 5:
 6:  namespace NiceCodeGenStylee
 7:  {
 8:      partial class Class1
 9:      {
10:          /// <summary>
11:          /// When this method in called the hand cranked
12:          /// code should have the implementation
13:          /// </summary>
14:          partial void Number2PropertyChanging(
                 IntReportChanges args)
15:          {
16:              if (args.Values.NewValue == 0)
17:              {
18:                  Console.WriteLine(
19:                      "Number2 is invalid value, cancelling");
20:                  args.Cancel = true;
21:              }
22:          }
23:
24:          /// <summary>
25:          /// When this method in called the hand cranked
26:          /// code should have the implementation
27:          /// </summary>
28:          partial void Number2CodePropertyChanged(
                 Int32ItemChanges args)
29:          {
30:              Console.WriteLine(String.Format(
31:                  "Number2 changed to {0}, doing calcs",
                     args.NewValue));
32:              DoNumberCalcs();
33:          }
34:
35:
36:          /// <summary>
37:          /// When this method in called the hand cranked
38:          /// code should have the implementation
39:          /// </summary>
40:          partial void Number1PropertyChanging(
                 IntReportChanges args)
41:          {
42:              if (args.Values.NewValue == 0)
43:              {
44:                  Console.WriteLine(
45:                      "Number1 is invalid value, cancelling");
46:                  args.Cancel = true;
47:              }
48:          }
49:
50:          /// <summary>
51:          /// When this method in called the hand cranked
52:          /// code should have the implementation
53:          /// </summary>
54:          partial void Number1CodePropertyChanged(
                 Int32ItemChanges args)
55:          {
56:              Console.WriteLine(String.Format(
57:                  "Number1 changed to {0}, doing calcs",
                     args.NewValue));
58:              DoNumberCalcs();
59:          }
60:
61:
62:          private void DoNumberCalcs()
63:          {
64:              Console.WriteLine(
65:                  String.Format("Number1 ({0}) + Number2 ({1}) ={2}",
66:                      number1, number2, number1 + number2));
67:          }
68:      }
69:  }

Where these little helper classes are used

 1:  using System;
 2:  using System.Collections.Generic;
 3:  using System.Linq;
 4:  using System.Text;
 5:
 6:  namespace NiceCodeGenStylee
 7:  {
 8:      struct Int32ItemChanges
 9:      {
10:          public readonly Int32 OldValue;
11:          public readonly Int32 NewValue;
12:
13:          public Int32ItemChanges(
                 Int32 oldValue, Int32 newValue)
14:          {
15:              OldValue = oldValue;
16:              NewValue = newValue;
17:          }
18:      }
19:
20:  }

 1:  using System;
 2:  using System.Collections.Generic;
 3:  using System.Linq;
 4:  using System.Text;
 5:
 6:  namespace NiceCodeGenStylee
 7:  {
 8:      class IntReportChanges
 9:      {
10:          public Int32ItemChanges Values { get; set; }
11:          public Boolean Cancel { get; set; }
12:      }
13:  }

And here is a demo of it running

See it all works lovely.