C# in Depth

Changes to type inference and overload resolution245C# query code withlambda expressionsC# query code withlambda expressionsCompile timeIL usingdelegatesC# compilerC# compilerIL usingexpression treesExecution timeQuery resultsDelegate codeexecuted directlyin the CLRDynamic SQLQuery resultsLINQ to SQL providerExecuted at databaseand fetched backLINQ to ObjectsLINQ to SQLFigure 9.4 Both LINQ to Objects and LINQ to SQL start off with C# code, and end with query results.The ability to execute the code remotely comes through expression trees.some of the expression locally and some elsewhere. We’ll look at some of the details ofthis conversion step in chapter 12, but you should bear this end goal in mind as weexplore extension methods and LINQ syntax in chapters 10 and 11.NOTENot all checking can be done by the compiler—When expression trees areexamined by some sort of converter, there are often cases that have to berejected. For instance, although it’s possible to convert a call tostring.StartsWith into a similar SQL expression, a call to string.IsInterned doesn’t make sense in a database environment. Expressiontrees allow a large amount of compile-time safety, but the compiler canonly check that the lambda expression can be converted into a validexpression tree; it can’t make sure that the expression tree will be suitablefor its eventual use.That finishes our direct coverage of lambda expressions and expression trees. Beforewe go any further, however, there are a few changes to C# that need some explanation,regarding type inference and how the compiler selects between overloaded methods.9.4 Changes to type inference and overload resolutionThe steps involved in type inference and overload resolution have been altered inC# 3 to accommodate lambda expressions and indeed to make anonymous methodsmore useful. This doesn’t count as a new feature of C# as such, but it can be importantto understand what the compiler is going to do. If you find details like thistedious and irrelevant, feel free to skip to the chapter summary—but remember thatthis section exists, so you can read it if you run across a compilation error related tothis topic and can’t understand why your code doesn’t work. (Alternatively, youmight want to come back to this section if you find your code does compile, but youdon’t think it should!)Licensed to Rhona Hadida