\nToday’s “Little Program” is really a\n“Little Puzzle” that got out of hand.\n<\/p>\n
\nThis started out as a practical question:\nThis code fragment screams out for some sort of simplification.\n(I’ve changed the names of the classes.)\n<\/p>\n
\nclass FrogProperty\n{\n public string Name { get; private set; }\n public string Value { get; private set; }\n ...\n}\nclass ToadProperty\n{\n public string Name { get; private set; }\n public string Value { get; private set; }\n ...\n}\nvar frogStuff = new List<string>();\nforeach (var frogProp in FrogProperties) {\n frogStuff.Add(string.Format(\"{0}: {1}\", frogProp.Name, frogProp.Value));\n}\nfrogStuff.Sort();\nMunge(frogStuff);\nvar toadStuff = new List<string>();\nforeach (var toadProp in ToadProperties) {\n toadStuff.Add(string.Format(\"{0} = {1}\", toadProp.Name, toadProp.Value));\n}\ntoadStuff.Sort();\nMunge(toadStuff);\nvar catStuff = new List<string>();\nforeach (var cat in Cats) {\n catStuff.Add(string.Format(\"{0}\", cat.Name));\n}\ncatStuff.Sort();\nMunge(catStuff);\nvar dogStuff = new List<string>();\nforeach (var dogProp in DogProperties) {\n dogStuff.Add(string.Format(\"{0} {1}\", dogProp.Name, dogProp.Value));\n}\ndogStuff.Sort();\nMunge(dogStuff);\n...\n<\/pre>\n\nClearly, the pattern is\n<\/p>\n
\nvar stuff = new List<string>();\nforeach (var thing in thingCollection) {\n stuff.Add(string.Format(formatstring, thing.Name, [optional: thing.Value]));\n}\nstuff.Sort();\nMunge(stuff);\n<\/pre>\n\nEverything here is pretty straightforward,\nexcept for the\nstring.Format<\/code> part.\nCan we write a function that takes a thing<\/code>\nand formats it in a somewhat flexible manner?\n<\/p>\n\nLet’s start with the\nName<\/code>-and-Value<\/code> cases.\nWe might try something like this:\n<\/p>\n\npublic static string FormatNameValue<T>(this T t, string format)\n{\n return string.Format(format, t.Name, t.Value);\n}\n<\/pre>\n\nBut then we’d run into trouble, because there is no constraint\non T<\/code>, so the compiler will complain,\n“I don’t know how to get a Name<\/code> or a\nValue<\/code> from an object<\/code>.”\n<\/p>\n\nAnd since\nFrogProperty<\/code>\nand\nToadProperty<\/code>\ndo not have a common base class,\nyou’re kind of stuck.\n<\/p>\n\nOne way out would be to use the new dynamic<\/code> type:\n<\/p>\n\npublic static string FormatNameValue<T>(this T t, string format)\n{\n dynamic d = t;\n return string.Format(format, d.Name, d.Value);\n}\n<\/pre>\n\nBut that won’t work in the Name<\/code>-only case:\n<\/p>\n\ncat.FormatNameValue(\"{0}\");\n<\/pre>\n\nThe cat<\/code> object has a\nName<\/code> but no Value<\/code>.\nThe attempt to read the Value<\/code> will raise an exception\n(even though it is never consumed by the format).\n<\/p>\n\nMaybe we can turn to reflection.\n<\/p>\n
\npublic static string FormatNameValue<T>(this T t, string format)\n{\n return string.Format(format,\n typeof(T).GetProperty(\"Name\").GetValue(t, null),\n typeof(T).GetProperty(\"Value\").GetValue(t, null));\n}\n<\/pre>\n\nThis still raises an exception if there is no Value<\/code>,\nbut we can detect the missing Value<\/code> before we run into\ntrouble with it.\n<\/p>\n\nstatic object GetPropertyOrNull<T>(this T t, string prop)\n{\n var propInfo = typeof(T).GetProperty(prop);\n return propInfo == null ? null : propInfo.GetValue(t, null);\n}\npublic static string FormatNameValue<T>(this T t, string format)\n{\n return string.Format(format,\n t.GetPropertyOrNull(\"Name\"),\n t.GetPropertyOrNull(\"Value\"));\n}\n<\/pre>\n\nOkay, now we’re getting somewhere.\n<\/p>\n
\nBut before getting to deep into this exercise, I should point\nout that another way to solve this problem is to turn it\ninside-out.\nInstead of making the munger understand all of the different\nobjects,\nwhy not make each object understand munging?\n<\/p>\n
\nclass FrogProperty : IFormattable\n{\n public string Name { get; private set; }\n public string Value { get; private set; }\n public override ToString(string format, IFormatProvider formatProvider)\n {\n switch (format) {\n case \"Munge\":\n return string.Format(formatProvider,\"{0}: {1}\", Name, Value);\n default:\n return ToString(); \/\/ use object.ToString();\n }\n }\n}\nclass Cat : IFormattable\n{\n public string Name { get; private set; }\n public override ToString(string format, IFormatProvider formatProvider)\n {\n switch (format) {\n case \"Munge\":\n return string.Format(formatProvider,\"{0}\", Name);\n default:\n return ToString(); \/\/ use object.ToString();\n }\n }\n}\n<\/pre>\n\nThe generic helper function would then be\n<\/p>\n
\nvar stuff = new List<string>();\nforeach (var thing in thingCollection) {\n stuff.Add(string.Format(\"{0:Munge}\", thing);\n}\nstuff.Sort();\nMunge(stuff);\n<\/pre>\n\nOkay, fine, rain on my little puzzle parade.\n<\/p>\n
\nLet’s ignore this very useful advice and proceed\nahead with our puzzle,\nbecause we’re determined to see how far we can go,\neven if it’s in the wrong direction<\/i>.\n<\/p>\n
\nNow that we have\nFormatNameValue<\/code>,\nwe might say,\n“What about generalizing to cases where we want\nproperties other than\nName<\/code> and Value<\/code>?”\nOne design would be to pass in a format string\nand list of properties you want to fill in:\n<\/p>\n\nthing.FormatProperties(\"{0}: {1} (modified by {2})\",\n \"Name\", \"Value\", \"ModifiedBy\");\n<\/pre>\n\nOur\nFormatNameValue<\/code>\nfunction would go something like this:\n<\/p>\n\npublic static string FormatProperties<T>(\n this T t, string format, params string[] props)\n{\n object[] values = new object[props.Length];\n for (var i = 0; i < props.Length; i++) {\n values[i] = typeof(T).GetProperty(props[i]).GetValue(t, null);\n }\n return string.Format(format, values);\n}\n<\/pre>\n\nThis suffers from a problem common to most formatters:\nOnce you get more than a few insertions, it becomes\nhard to figure out which one matches up to what.\nSo I’m going to try something radical:\n<\/p>\n
\nstatic Regex identifier = new Regex(@\"(?<={)(.*?)(?=[:}])\");\n\/\/ pedants<\/a> would use\n\/\/identifier = new RegEx(@\"[_\\p{Lu}\\p{Ll}\\p{Lt}\\p{Lm}\\p{Lo}\\p{Nl}]\" +\n\/\/ @\"[_\\p{Lu}\\p{Ll}\\p{Lt}\\p{Lm}\\p{Lo}\\p{Nl}\\d\\p{Pc}\\p{Mn}\\p{Mc}]\");\npublic static string FormatProperties<T>(this T t, string format)\n{\n var values = new ArrayList();\n int count = 0;\n format = identifier.Replace(format, (m) => {\n values.Add(typeof(T).GetProperty(m.Value).GetValue(t, null));\n return (count++).ToString();\n });\n return string.Format(format, values.ToArray());\n}\n<\/pre>\n\nInstead of separating the properties from the format,\nI embed them in the format.\n<\/p>\n
\nthing.FormatProperties(\"{Name}: {Value} (modified by {ModifiedBy})\");\n<\/pre>\n\nNote that I explicitly exclude colons from identifiers.\nThat lets me do things like this:\n<\/p>\n
\nvar result =\n (new System.IO.FileInfo(@\"C:\\Windows\\Explorer.exe\"))\n .FormatProperties(\"Created on {CreationTime:F} \" +\n \"{Length} bytes in size\");\n<\/pre>\n\nThe property names are extracted and replaced with corresponding\nnumbers, but the format string remains,\nallowing it to be used to alter the final formatting of the property.\n<\/p>\n
\nOkay, at this point I figured I had gone far enough.\nThe fun had run out,\nso I decided to stop.<\/p>\n","protected":false},"excerpt":{"rendered":"
Today’s “Little Program” is really a “Little Puzzle” that got out of hand. This started out as a practical question: This code fragment screams out for some sort of simplification. (I’ve changed the names of the classes.) class FrogProperty { public string Name { get; private set; } public string Value { get; private set; […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-43653","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Today’s “Little Program” is really a “Little Puzzle” that got out of hand. This started out as a practical question: This code fragment screams out for some sort of simplification. (I’ve changed the names of the classes.) class FrogProperty { public string Name { get; private set; } public string Value { get; private set; […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/43653","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=43653"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/43653\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=43653"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=43653"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=43653"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}