Use tabs, not spaces.

BuildTools/tidy.py

@@ -0,0 +1,30 @@
+import os, sys
+
+def check(filename):
+	ok = True
+	with open(filename, 'r') as f:
+		for i, line in enumerate(f):
+			if line.startswith(' '):
+				print('{}:{}: Line starting with spaces. Use tabs for indentation instead!'.format(filename, i+1))
+				ok = False
+	return ok
+
+def main():
+	root_dir = os.path.normpath(os.path.join(os.path.dirname(__file__), '..'))
+	dirs_to_check = (
+		os.path.join(root_dir, subdir)
+		for subdir in ('ICSharpCode.Decompiler', 'ILSpy', 'ILSpy.BamlDecompiler'))
+	ok = True
+	for dir in dirs_to_check:
+		for root, dirs, files in os.walk(dir):
+			if '\\obj\\' in root:
+				continue
+			for filename in files:
+				if filename.lower().endswith('.cs'):
+					if not check(os.path.join(root, filename)):
+						ok = False
+	print('Tidy check: {}'.format('successful' if ok else 'failed'))
+	return 0 if ok else 1
+
+if __name__ == '__main__':
+	sys.exit(main())

appveyor.yml

@@ -9,3 +9,5 @@ build:
   project: ILSpy.sln
   parallel: true
   verbosity: minimal
+after_test:
+- python ICSharpCode.AvalonEdit.Tests\tidy.py