Looking at the code, that's pretty smart.
The magic is happening in the ActualCodeTextGetter class. First, it retrieves the line of the source code file by using the StackTrace:
StackTrace stackTrace = trace ?? new StackTrace(true);
// Cut for brevity
StackFrame stackFrame = frame;
this.ShouldlyFrameIndex = index - 1;
string fileName = stackFrame.GetFileName();
this._determinedOriginatingFrame = fileName != null && File.Exists(fileName);
this._shouldMethod = this.ShouldlyFrame.GetMethod().Name;
this.FileName = fileName;
this.LineNumber = stackFrame.GetFileLineNumber() - 1;
Once it has the name of the source code file, along with the line and offset of the statement, it's just a matter of reading directly the file:
private string GetCodePart()
{
string str = "Shouldly uses your source code to generate its great error messages, build your test project with full debug information to get better error messages\nThe provided expression";
if (this._determinedOriginatingFrame)
{
string codeLines = string.Join("\n", ((IEnumerable<string>) File.ReadAllLines(this.FileName)).Skip<string>(this.LineNumber).ToArray<string>());
int indexOfMethod = codeLines.IndexOf(this._shouldMethod);
if (indexOfMethod > 0)
str = codeLines.Substring(0, indexOfMethod - 1).Trim();
str = !str.EndsWith("Should") ? str.RemoveVariableAssignment().RemoveBlock() : this.GetCodePartFromParameter(indexOfMethod, codeLines, str);
}
return str;
}
There's a lot more logic going on to isolate precisely the statement, but in short the trick is:
- Use the StackTrace to retrieve the location of the source code and the line of the statement
- Parse the source code to retrieve the exact statement
Of course, it can work only if you're running it on the same machine you used to compile the code.
