We\u00a0hit a little speed bump\u00a0in our\u00a0work with\u00a0parameter-less\u00a0lambdas\u00a0– we\u00a0wanted to be able to assign them to objects of a custom class via an implicit cast, this is very important to our cunning plan…<\/p>\n
\u00a0<\/p>\n
To illustrate this consider the following class that just encapsulates a 0-arity delegate Func<T>:<\/p>\n
\u00a0\u00a0\u00a0 public class RuleProp<T>\r\n\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 private Func<T> _lambda = null;<\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 public RuleProp(Func<T> l)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 _lambda = l;\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }<\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 public static implicit operator RuleProp<T>(Func<T> l)\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return new RuleProp<T>(l);\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }<\/pre>\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 public RuleProp()\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 }\r\n\u00a0\u00a0\u00a0 }<\/pre>\nNow, because we have specified an implicit cast for: Func<T> -> RuleProp<T>, it would seem \u00a0that we should be able to do the following:<\/p>\n
var rp = new RuleProp<string>();\r\nrp = () => \"hello\"; \/\/ Not OK, can't convert lambda to delegate\u00a0<\/pre>\nBut this gives us a compiler error, can’t convert lambda to Fun<T> as it’s not a delegate!!\u00a0 Looking at the C# specification it seems that if more than one implicit conversion is required, the C# compiler will only do one.\u00a0 In this case 2 conversions are required:<\/p>\n
\u00a0<\/p>\n
Lambda Expression -> Func<T> -> RuleProp<T><\/p>\n
\u00a0<\/p>\n
And so the compiler can’t handle it.\u00a0 If we provide an explicit cast to Func<T> it compiles OK:<\/p>\n
\u00a0<\/p>\n
rp = (Func<string>)(() => \"hello\");\u00a0 \/\/ Cast works..<\/pre>\n\u00a0<\/p>\n
But this is really ugly as the type must be provided each time.\u00a0 A workaround was suggested in this article:<\/p>\n
\u00a0<\/p>\n
http:\/\/www.interact-sw.co.uk\/iangblog\/2008\/03\/17\/lambda-inference<\/span><\/a><\/p>\n\u00a0<\/p>\n
We provide an ‘adaptor’ function which encourages a cast from Lambda -> Func<T>, and the implicit cast from Func<T> to RuleProp<T> will handle the rest, we can use it like this:<\/p>\n
\u00a0<\/p>\n
rp = Adaptor(() => \"hello\");\u00a0 \/\/ Now OK!<\/pre>\nThis is much nicer than the explicit cast as no type needs to be specified!\u00a0 Here is the function, it just take,s and directly returns a delegate –\u00a0resulting in just the conversion that we need!<\/p>\n
\u00a0<\/p>\n
public static Func<T> Adaptor<T>(Func<T> f)\r\n{\r\n\u00a0return f;\r\n}<\/pre>\n\u00a0<\/p>\n
So although it is not as nice as directly assigning a lambda to RuleProp<T> it’s a grand workaround and will do nicely for the time being – thanks to all at the C# MSDN<\/a> forum for their help!<\/p>\n","protected":false},"excerpt":{"rendered":"We\u00a0hit a little speed bump\u00a0in our\u00a0work with\u00a0parameter-less\u00a0lambdas\u00a0– we\u00a0wanted to be able to assign them to objects of a custom class via an implicit cast, this is very important to our cunning plan… \u00a0 To illustrate this consider the following class that just encapsulates a 0-arity delegate Func<T>: \u00a0\u00a0\u00a0 public class RuleProp<T> \u00a0\u00a0\u00a0 { \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 private […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[11,12],"class_list":["post-40","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-cplusplus","tag-lambda-expression"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t