{"id":96151,"date":"2023-09-26T06:52:58","date_gmt":"2023-09-26T04:52:58","guid":{"rendered":"https:\/\/code-maze.com\/?p=96151"},"modified":"2023-09-26T06:52:58","modified_gmt":"2023-09-26T04:52:58","slug":"csharp-how-to-use-discard-variable","status":"publish","type":"post","link":"https:\/\/code-maze.com\/csharp-how-to-use-discard-variable\/","title":{"rendered":"Using a Discard Variable in C#"},"content":{"rendered":"

In this article, we are going to learn about the differences between a discard variable and the usual variables in C#. We will also see some practical uses of a discard.\u00a0<\/p>\n

To download the source code for this article, you can visit our GitHub repository<\/a>.<\/div>\n

C# 7 introduced discards as placeholders for values we do not have any use for in our code<\/strong>. They are write-only variables that allow us to explicitly inform the compiler or anyone reading our code to discard their content. They are particularly useful in scenarios where we receive a value but have no intention of using it.<\/p>\n

How a Discard Variable Differs From a Usual Variable<\/h2>\n

A discard is represented by the underscore (_<\/code>) character:<\/p>\n

_ = \"This is the syntax for a discard\";<\/code><\/p>\n

Discards are like unassigned variables. But unlike normal variables, we cannot declare discards with var<\/code> or any type keyword.<\/p>\n

We can have more than one discard within a scope. We use each underscore placeholder to discard a single value. Even though it looks like discards have values, when we attempt to retrieve any of them, we get a compiler error: CS0103: The name '_' doesn't exist in the current context<\/code>.\u00a0<\/p>\n

We should note that declaring an underscore with a var<\/code> or other types makes it a regular variable and loses its discard functionality:<\/strong><\/p>\n

string _ = \"This is no longer a discard\";<\/code><\/p>\n

Why Do We Use a Discard Variable?<\/h2>\n

If we ignore the discards, why do we need them?<\/p>\n

Discards help us maintain a clean codebase with minimized warnings. When we call a method with a return value we do not care for, assigning that value to a discard takes care of compiler warnings about unused variables. Some APIs and libraries also return unwanted values. We employ discards to manage them and still conform to the API\/library signatures.<\/p>\n

When working with pattern matching and deconstruction, discards allow us to ignore parts of a data structure that are irrelevant to us.\u00a0<\/p>\n

They are a feature that contributes to simplifying our codes and improving clarity.<\/p>\n

Use Cases for Using a Discard Variable in C#<\/h2>\n
\n

Using a discard in our code effectively communicates our intention to the compiler and other code readers. It signifies that we deliberately intend to disregard the outcome of an expression.<\/p>\n

We could ignore the result of an expression, one or more elements within a tuple expression, an out<\/code> parameter in a method or the target of a pattern-matching expression.<\/p>\n

Let’s explore various scenarios where discards come in handy.<\/p>\n

Discards as Out Parameters<\/h3>\n

We can discard the value of an out<\/code> parameter when calling any method that uses one.\u00a0<\/p>\n

For example, a TryParse()<\/code> returns a bool<\/code>. If the parse is successful, it returns true and saves the value of the parsed argument to the out<\/code> parameter:<\/p>\n

public static (bool result, string errorMessage) GetNumber(string input)\r\n{\r\n    bool result = int.TryParse(input, out _);\r\n\r\n    if (!result)\r\n    {\r\n        return (false, \"Not a valid number\");\r\n    }\r\n\r\n    return (true, \"\");\r\n}<\/pre>\n
In our example, we use int.TryParse()<\/code> to attempt to parse an input, which is a string<\/code> by default. We use discard for the out<\/code> parameter to indicate that we are not interested in the parsed integer value. We are only checking if the parse was successful or not.\u00a0<\/p>\n
Discards in Tuple Deconstruction<\/h3>\n
When working with tuples<\/a>, we often do not require all the values they contain. So, let’s start with a simple method:<\/p>\n
public static (int, int, int) GetSum(int num1, int num2)\r\n{\r\n    return (num1, num2, num1 + num2);\r\n}<\/pre>\n
The GetSum()<\/code> method returns a three-tuple containing the numbers to be added and the resulting sum.\u00a0<\/p>\n
Using a discard, we can selectively ignore the specific tuple elements<\/strong> we want and use actual identifiers for the values we care about:<\/p>\n
var (_, _, sum) = DiscardExamples.GetSum(9, 89);<\/code><\/p>\n
Since the two numbers are already passed into GetSum()<\/code> when we call it, we do not need their values returned. Our interest is obtaining the sum, so we designate the other two numbers as discards.<\/p>\n
Discards in Pattern Matching<\/h3>\n
Given an array of items containing various data types, we can use pattern matching<\/a> within a switch<\/code> expression to identify the type of each item:<\/p>\n
public static void GetType(object[] objects)\r\n{\r\n    foreach (var item in objects)\r\n    {\r\n        Console.WriteLine(item switch\r\n        {\r\n            string => \"it's a string\",\r\n            int => \"it's an int\",\r\n            _ => \"Neither string nor int\"\r\n        });  \r\n    }\r\n}<\/pre>\n
Our method checks if the items in our array are a string<\/code> or an integer<\/code>.\u00a0 We treat any other data type as a discard, so the method will print Neither string nor int<\/code>\u00a0to the console.<\/p>\n
Discards in Lambda Expressions<\/h3>\n
From C# 9, we can use discards to represent two or more unused input parameters of a lambda expression<\/a>:<\/p>\n
Func<char, char, char> constant = (_, _) => 'C';<\/code><\/p>\n
To maintain compatibility for lambda expressions written in older versions of C#, when a single input parameter is assigned the _<\/code> identifier, it is interpreted as a valid read\/write variable in newer versions.<\/p>\n
Conclusion<\/h2>\n
Using the discard _<\/code> in our code conveys to the compiler that we mean to disregard the outcome of an expression deliberately. It proves especially handy in situations where we receive a value but have no plans to utilize it.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
In this article, we are going to learn about the differences between a discard variable and the usual variables in C#. We will also see some practical uses of a discard.\u00a0 C# 7 introduced discards as placeholders for values we do not have any use for in our code. They are write-only variables that allow […]<\/p>\n","protected":false},"author":6,"featured_media":62189,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_et_pb_use_builder":"","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[12,506],"tags":[1811,1963],"class_list":["post-96151","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-csharp","category-intermediate","tag-c","tag-discard","et-has-post-format-content","et_post_format-et-post-format-standard"],"yoast_head":"\nUsing a Discard Variable in C# - Code Maze<\/title>\n<meta name=\"description\" content=\"In this article, we will learn the difference between a discard variable and the usual variables in C#. 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