Build your own debugger using Roslyn's syntax analysis

// This is the 23rd article of C# Advent Calendar 2018 in Japan.
// The original article is Roslyn の構文解析を使ってデバッガーを自作する.

I tried to make something like a debugger.

Motivation

• I normally use Visual Studio, but it is troublesome to step through code manually at debugging
  • When the number of steps is large in a loop or the like
  • When I want to check the state of branching or variables lightly

Solution

• Specify only the time interval of the step and let the debugger run automatically
  • A list of variables is displayed
  • Time interval can be adjusted in real time
• Using the syntax analysis of .NET Compiler Platform (Roslyn), it seems to be realizable if we insert debugging code between each step

Result

So, as a result of trying to make prototype "Tick-tack Debugger" with WPF, it looked like this.
As an example, we are figuring out a square root by the Newton's method.

Commentary

The following is an outline technical explanation.
Before creating a WPF application, first try a console application on the .NET Framework.
To use C# syntax analysis, install Microsoft.CodeAnalysis.CSharp via NuGet.

It is a policy to insert debugging code into the source code to be debugged, compile it dynamically and execute it.
The source code of the console application is shown below (the whole solution is in SyntaxTreeSample).

In the SyntaxHelper class, C# source code to be debugged is converted to a syntax tree and scanned, and a line of code for debugging is inserted before each statement.

By using the CSharpSyntaxTree.ParseText method, you can convert the source code to a SyntaxTree object.
Also, the superclass representing all nodes, such as methods, statements and expressions, is the SyntaxNode class,

• Parent property
• Ancestors method
• ChildNodes method
• DescendantNodes method

if you know these members, you can scan nodes.

In addition to this, DebuggerLib is created as a class library that defines methods called from code for debugging.
Let this library go through to notify the position of each statement and, the variables and their values that exist immediately before it.

In the Program class, after saving the generated debugging source code to a file, compile it using CodeDomProvider in the System.CodeDom.Compiler namespace, and call its entry point (the Main method).
It also registers the event handler when the debug code is executed, and pauses the program for the specified time using the Thread.Sleep method.

Now, if the original source code to be debugged is Program.cs below, the following Program.g.cs is generated as the debugging source code.

When we run the console application we created, it will look like the following figure (the time interval is 0.3 seconds).

Based on the above, we created a debugging tool as a WPF application.
The part of the C# source code on the left is a TextBox and can be edited.
When debugging is executed, highlighting is done by setting each statement to the selected state.
The part where the variable list is displayed on the right is a DataGrid.

(An example of figuring out the pi)

This time I tried to make a prototype by the above method, but I think that there is a smarter way to insert debug code and compile.

Remarks

• We can not deal with all possible statements. Also, we only scan the Main method.
• The assemblies generated at compile time (EXE) are saved in the %TEMP% folder (user's AppData\Local\Temp).
• In the TextBox, setting IsInactiveSelectionHighlightEnabled to True may not work. In addition, highlights in the selected state may slip. It might be better to use Run etc in RichTextBox.

Samples Created

• SyntaxTreeSample (GitHub)

Version

• .NET Framework 4.7
• Microsoft.CodeAnalysis.CSharp 2.10.0
• ReactiveProperty 5.3.2

References

• .NET Compiler Platform SDK (Roslyn API)