Revisions to lambda expressions topic. #1285
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❌ Validation status: errorsdocs/csharp/lambda-expressions.md
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There are some build errors. It would be good to fix those first, otherwise we can't even see it on staging. |
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@mairaw I think I've fixed all the build errors. A new build should run shortly. |
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| keywords: .NET, .NET Core | ||
| author: dotnet-bot | ||
| manager: wpickett | ||
| ms.date: 06/20/2016 |
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Update the date; also needs a ms.author.
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@stevehoag Thanks. I completely forgot to copy the header from the original topic.
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| ## Lambda expressions and tuples ## | ||
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| Starting with C# 7.0, the C# language provides built-in support for tuples. You can provide a tuple as an argument to a lambda expression, and your lambda expression can also return a tuple. In some cases, the C# compiler uses type inference to determine the types of tuple components. |
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Link to the Tuples topic?
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@stevehoag I've added it. Thanks for thinking of it; I'd intended to do it, and forgot
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| [!CODE [csSnippets.Lambdas](../../samples/snippets/csharp/concepts/lambda-expressions/query1.cs#3)] | ||
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| You can also supply a lambda expression when the argument type is an @System.Linq.Expressions.Expression%601, for example in the standard query operators that are defined in the @System.Linq.Queryable type. When you specify an @System.Linq.Expressions.Expression%601 argument, the lambda is compiled to an expression tree. The following example uses the @System.Linq.Enumerable.Count%60%601(System.Collections.Generic.IEnumerable{t%60%600})) standard query operator. |
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The last link (@System.Linq.Enumerable.Count) isn't resolving.
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| ## See Also ## | ||
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| [LINQ (Language-Integrated Query)](../standard/using-linq.md) |
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These three links are all showing up on a single line - was that intentional?
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| - A lambda expression cannot contain a `goto` statement, `break` statement, or `continue` statement that is inside the lambda function if the jump statement’s target is outside the block. It is also an error to have a jump statement outside the lambda function block if the target is inside the block. | ||
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| ## Featured Book Chapter ## |
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Should we still be linking to this? The book was published in 2007.
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| [!CODE [csSnippets.Lambdas](../../samples/snippets/csharp/concepts/lambda-expressions/infer1.cs#1)] | ||
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| The general rules for type inteference for lambdas are: |
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@mairaw And I didn't even spell the wrong word (interference) correctly. Fixed.
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| ## Variable Scope in Lambda Expressions ## | ||
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| Lambdas can refer to *outer variables* (see [Anonymous methods](programming-guide/statements-expressions-operators/anonymous-methods.md)) that are in scope in the method that defines the lambda function, or in scope in the type that contains the lambda expression. Variables that are captured in this manner are stored for use in the lambda expression even if the variables would otherwise go out of scope and be garbage collected. An outer variable must be definitely assigned before it can be consumed in a lambda expression. The following example demonstrates these rules. |
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must be definitely -> must be (I think the must is enough)
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No, this should stay "definitely assigned". That's the phrase the spec always uses for the analysis of assignment to a variable before it is used.
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| ## Type inference in lambda expressions ## | ||
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| When writing lambdas, you often do not have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the parameter’s delegate type, and other factors, as described in the C# Language Specification. For most of the standard query operators, the first input is the type of the elements in the source sequence. So if you are querying an `IEnumerable<Customer>`, then the input variable is inferred to be a `Customer` object, which means you have access to its methods and properties: |
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nit: missing comma after So
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I think you should delete "delegate" in the phrase "the parameter's delegate type". Or am I misunderstanding this sentence?
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@mairaw @BillWagner I've removed the "So". And changed "the parameter's delegate type" to "the parameter types".
| ms.assetid: b6a0539a-8ce5-4da7-adcf-44be345a2714 | ||
| -title: Lambda Expressions | ||
| -description: Lean to use lambda expressions, which are executable code blocks that can be passed as arguments. | ||
| -keywords: .NET, .NET Core, lambda epxressions, lambdas, delegates |
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| - Creating [expression trees](expression-trees-building.md). | ||
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| Lambda expressions are delegates. The specific delegate type of a lambda expressions depends on its parameters and return value. Lambda expressions that don't return a value correspond to a specific `Action` delegate, depending on its number of parameters. Lambda expressions that return a value correspond to a specific `Func` delegate, depending on its number of parameters. For example, a lambda expression that has two parameters but returns no value corresponds to an @System.Action%602 delegate. A lambda expression that has one parameter and returns a value corresponds to @System.Func%602 delegate. |
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Lambda expressions are delegates, except when they are expression trees.
Maybe "Lambda expressions are code that can be represented either as a delegate, or an expression tree that would compile to a delegate". (I'm not sure I like that wording, improve at will.)
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| [!CODE [csSnippets.Lambdas](../../samples/snippets/csharp/concepts/lambda-expressions/expression3.cs#3)] | ||
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| Note in the previous example that the body of an expression lambda can consist of a method call. However, if you are creating expression trees that are evaluated outside of the .NET Framework, such as in SQL Server, you should not use method calls in lambda expressions. The methods will have no meaning outside the context of the .NET runtime. |
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We should add some weasel words here. Entity Framework does translate some known methods (especially on the string class. The guidance is correct in the general case.
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| [!CODE [csSnippets.Lambdas](../../samples/snippets/csharp/concepts/lambda-expressions/statement1.cs#1)] | ||
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| Statement lambdas, like anonymous methods, cannot be used to create expression trees. |
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I'd recommend deleting "like anonymous methods". The C# spec defines lambda expressions as a syntax for anonymous methods, so in that sense all lambda expressions are anonymous methods.
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| ## Lambdas with the standard query operators ## | ||
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| Many standard query operators have an input parameter whose type is one of the @System.Func%601 family of generic delegates. These delegates use type parameters to define the number and type of input parameters, and the return type of the delegate. `Func` delegates are very useful for encapsulating user-defined expressions that are applied to each element in a set of source data. For example, consider the @System.Func%601 delegate, whose syntax is: |
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I would start this by saying "LINQ to Objects, among other implementations" have an input parameter..
That way this is distinguished from the IQueryable discussion later.
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| ## Type inference in lambda expressions ## | ||
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| When writing lambdas, you often do not have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the parameter’s delegate type, and other factors, as described in the C# Language Specification. For most of the standard query operators, the first input is the type of the elements in the source sequence. So if you are querying an `IEnumerable<Customer>`, then the input variable is inferred to be a `Customer` object, which means you have access to its methods and properties: |
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| ## Type inference in lambda expressions ## | ||
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| When writing lambdas, you often do not have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the parameter’s delegate type, and other factors, as described in the C# Language Specification. For most of the standard query operators, the first input is the type of the elements in the source sequence. So if you are querying an `IEnumerable<Customer>`, then the input variable is inferred to be a `Customer` object, which means you have access to its methods and properties: |
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I think you should delete "delegate" in the phrase "the parameter's delegate type". Or am I misunderstanding this sentence?
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| ## Variable Scope in Lambda Expressions ## | ||
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| Lambdas can refer to *outer variables* (see [Anonymous methods](programming-guide/statements-expressions-operators/anonymous-methods.md)) that are in scope in the method that defines the lambda function, or in scope in the type that contains the lambda expression. Variables that are captured in this manner are stored for use in the lambda expression even if the variables would otherwise go out of scope and be garbage collected. An outer variable must be definitely assigned before it can be consumed in a lambda expression. The following example demonstrates these rules. |
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No, this should stay "definitely assigned". That's the phrase the spec always uses for the analysis of assignment to a variable before it is used.
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@BillWagner I've addressed all of your comments and will update the topic. |
✅ Validation status: passed
For more details, please refer to the build report. Note: If you changed an existing file name or deleted a file, broken links in other files to the deleted or renamed file are listed only in the full build report. |
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@rpetrusha LGTM. |
Revisions to lambda expressions topic
Summary
Extensively revised with new examples and a section on tuples and lambda expressions.
Suggested Reviewers
@BillWagner @stevehoag @mairaw