{"id":3502,"date":"2025-04-13T13:21:28","date_gmt":"2025-04-13T05:21:28","guid":{"rendered":"https:\/\/www.laixuexila.com\/?p=3502"},"modified":"2025-04-13T13:21:28","modified_gmt":"2025-04-13T05:21:28","slug":"c-%e5%8f%af%e7%a9%ba%e7%b1%bb%e5%9e%8b%ef%bc%88nullable-types%ef%bc%89","status":"publish","type":"post","link":"https:\/\/www.laixuexila.com\/index.php\/2025\/04\/13\/c-%e5%8f%af%e7%a9%ba%e7%b1%bb%e5%9e%8b%ef%bc%88nullable-types%ef%bc%89\/","title":{"rendered":"C# \u53ef\u7a7a\u7c7b\u578b\uff08Nullable Types\uff09"},"content":{"rendered":"\n<p>\u4e0b\u9762\u662f\u5bf9 <strong>C# \u53ef\u7a7a\u7c7b\u578b\uff08Nullable Types\uff09<\/strong> \u7684\u5168\u9762\u4e13\u4e1a\u8bb2\u89e3\uff0c\u4fe1\u606f\u6765\u6e90\u4e8e <a href=\"https:\/\/learn.microsoft.com\/en-us\/dotnet\/csharp\/language-reference\/builtin-types\/nullable-value-types\">Microsoft \u5b98\u65b9\u6587\u6863<\/a>\u3001.NET \u5b9e\u8df5\u7ecf\u9a8c\uff0c\u5e76\u63d0\u4f9b\u591a\u79cd\u4f7f\u7528\u65b9\u5f0f\u548c\u5b9e\u7528\u5efa\u8bae\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e00\u3001\u4ec0\u4e48\u662f\u53ef\u7a7a\u7c7b\u578b\uff1f<\/h2>\n\n\n\n<p>\u5728 C# \u4e2d\uff0c<strong>\u503c\u7c7b\u578b\uff08\u5982 int\u3001bool\u3001DateTime \u7b49\uff09\u9ed8\u8ba4\u4e0d\u80fd\u4e3a null<\/strong>\u3002\u800c\u6709\u65f6\u6211\u4eec\u9700\u8981\u8868\u793a\u4e00\u4e2a\u201c\u672a\u8d4b\u503c\u201d\u6216\u201c\u7f3a\u5931\u201d\u7684\u72b6\u6001\uff0c\u8fd9\u65f6\u5c31\u53ef\u4ee5\u4f7f\u7528 <strong>\u53ef\u7a7a\u7c7b\u578b<\/strong>\u3002<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\u2705 \u53ef\u7a7a\u7c7b\u578b\u5141\u8bb8\u503c\u7c7b\u578b\u53d8\u91cf\u53ef\u4ee5\u5b58\u50a8 <strong>null \u503c<\/strong>\u3002<\/p>\n<\/blockquote>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e8c\u3001\u8bed\u6cd5\u5b9a\u4e49<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>int? x = null;        \/\/ \u53ef\u7a7a int \u7c7b\u578b\nbool? isReady = true; \/\/ \u53ef\u7a7a bool \u7c7b\u578b<\/code><\/pre>\n\n\n\n<p>\u7b49\u4ef7\u5199\u6cd5\uff08\u4f7f\u7528 <code>Nullable&lt;T&gt;<\/code>\uff09\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>Nullable&lt;int&gt; x = null;<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e09\u3001\u9002\u7528\u4e8e\u54ea\u4e9b\u7c7b\u578b\uff1f<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\u53ea\u80fd\u7528\u4e8e <strong>\u503c\u7c7b\u578b\uff08value types\uff09<\/strong><\/li>\n\n\n\n<li>\u5f15\u7528\u7c7b\u578b\u672c\u8eab\u5c31\u53ef\u4ee5\u4e3a <code>null<\/code>\uff0c\u65e0\u9700\u53ef\u7a7a<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">\u2705 \u652f\u6301\u7684\u5e38\u7528\u503c\u7c7b\u578b\uff1a<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>int?       \/\/ Nullable&lt;int&gt;\ndouble?    \/\/ Nullable&lt;double&gt;\nbool?      \/\/ Nullable&lt;bool&gt;\nDateTime?  \/\/ Nullable&lt;DateTime&gt;\nenum?      \/\/ Nullable&lt;YourEnum&gt;<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u56db\u3001\u4f7f\u7528 Nullable \u7c7b\u578b\u7684\u5e38\u89c1\u64cd\u4f5c<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">1. \u5224\u65ad\u662f\u5426\u6709\u503c<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>int? x = null;\nif (x.HasValue)\n{\n    Console.WriteLine(x.Value);\n}\nelse\n{\n    Console.WriteLine(\"x is null\");\n}<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">2. \u83b7\u53d6\u503c\u6216\u9ed8\u8ba4\u503c\uff08Null \u5408\u5e76\u8fd0\u7b97\u7b26 <code>??<\/code>\uff09<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>int? x = null;\nint y = x ?? -1; \/\/ \u5982\u679c x \u4e3a null\uff0c\u5219 y = -1<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">3. \u62c6\u7bb1 nullable\uff08Null \u5408\u5e76\u8d4b\u503c <code>??=<\/code>\uff09<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>int? number = null;\nnumber ??= 100; \/\/ \u5982\u679c number \u4e3a null\uff0c\u5c31\u8d4b\u503c 100\nConsole.WriteLine(number); \/\/ \u8f93\u51fa 100<\/code><\/pre>\n\n\n\n<h3 class=\"wp-block-heading\">4. \u53ef\u7a7a\u7c7b\u578b\u8f6c\u6362\uff08\u663e\u5f0f\/\u9690\u5f0f\uff09<\/h3>\n\n\n\n<pre class=\"wp-block-code\"><code>int? x = 5;\ndouble? d = x; \/\/ \u9690\u5f0f\u8f6c\u6362<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e94\u3001Nullable \u4e0e\u6761\u4ef6\u8fd0\u7b97\u7b26\uff08?.\uff09<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>DateTime? date = GetDateOrNull();\nint? year = date?.Year;<\/code><\/pre>\n\n\n\n<p>\u89e3\u91ca\uff1a\u53ea\u6709\u5f53 <code>date<\/code> \u4e0d\u4e3a null\uff0c\u624d\u8bbf\u95ee <code>Year<\/code> \u5c5e\u6027\uff0c\u5426\u5219\u7ed3\u679c\u4e3a null\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u516d\u3001\u8bbf\u95ee <code>Value<\/code> \u65f6\u7684\u98ce\u9669<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>int? x = null;\nint y = x.Value; \/\/ \u274c \u629b\u51fa InvalidOperationException<\/code><\/pre>\n\n\n\n<p>\u5efa\u8bae\u7528 <code>HasValue<\/code> \u6216 <code>??<\/code> \u8fd0\u7b97\u7b26\u5904\u7406 null \u5b89\u5168\u6027\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e03\u3001Nullable \u4e0e\u6570\u636e\u5e93\u573a\u666f\uff08ADO.NET\uff09<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>SqlDataReader reader = command.ExecuteReader();\nint? age = reader.IsDBNull(0) ? null : reader.GetInt32(0);<\/code><\/pre>\n\n\n\n<p>\u5728\u6570\u636e\u5e93\u4e2d\uff0c\u5f88\u591a\u5b57\u6bb5\u5141\u8bb8 NULL\uff0c\u4f7f\u7528 Nullable \u53ef\u4ee5\u65e0\u7f1d\u5339\u914d SQL NULL \u503c\u3002<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u516b\u3001\u4e0e\u7ed3\u6784\u4f53\u548c\u679a\u4e3e\u914d\u5408\u4f7f\u7528<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>enum Status { Pending, Done, Failed }\n\nStatus? taskStatus = null;\nif (taskStatus == null)\n{\n    Console.WriteLine(\"Status unknown.\");\n}<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u4e5d\u3001\u793a\u4f8b\uff1a\u4f7f\u7528 Nullable \u7c7b\u578b\u8fdb\u884c\u4e1a\u52a1\u903b\u8f91\u5224\u65ad<\/h2>\n\n\n\n<pre class=\"wp-block-code\"><code>public decimal? GetDiscount(DateTime? birthDate)\n{\n    if (!birthDate.HasValue)\n        return null;\n\n    int age = DateTime.Now.Year - birthDate.Value.Year;\n    return age &gt;= 60 ? 0.1m : 0.0m;\n}<\/code><\/pre>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u5341\u3001\u5b98\u65b9\u53c2\u8003\u94fe\u63a5<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\ud83d\udd17 <a href=\"https:\/\/learn.microsoft.com\/en-us\/dotnet\/csharp\/language-reference\/builtin-types\/nullable-value-types\">C# \u53ef\u7a7a\u503c\u7c7b\u578b\uff08Microsoft Learn\uff09<\/a><\/li>\n\n\n\n<li>\ud83d\udd17 <a href=\"https:\/\/learn.microsoft.com\/en-us\/dotnet\/api\/system.nullable-1?view=net-8.0\">Nullable \u7ed3\u6784\u6587\u6863\uff08.NET API Reference\uff09<\/a><\/li>\n\n\n\n<li>\ud83d\udd17 <a href=\"https:\/\/learn.microsoft.com\/en-us\/dotnet\/csharp\/language-reference\/operators\/null-coalescing-operator\">C# Null \u5408\u5e76\u8fd0\u7b97\u7b26 ??<\/a><\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u5341\u4e00\u3001\u6700\u4f73\u5b9e\u8df5\u5efa\u8bae<\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th>\u5efa\u8bae<\/th><th>\u8bf4\u660e<\/th><\/tr><\/thead><tbody><tr><td>\u2705 \u5408\u7406\u4f7f\u7528 <code>??<\/code> \u6216 <code>HasValue<\/code><\/td><td>\u907f\u514d\u8bbf\u95ee <code>.Value<\/code> \u65f6\u629b\u51fa\u5f02\u5e38<\/td><\/tr><tr><td>\u2705 \u591a\u7528\u4e8e\u6570\u636e\u5e93\u3001\u8868\u5355\u7b49\u53ef\u7f3a\u7701\u8f93\u5165\u7684\u6570\u636e\u573a\u666f<\/td><td>\u53ef\u8bfb\u6027\u9ad8\uff0c\u8868\u793a\u660e\u786e\u610f\u56fe<\/td><\/tr><tr><td>\u274c \u4e0d\u63a8\u8350\u5728\u5f15\u7528\u7c7b\u578b\u4e0a\u518d\u7528 <code>Nullable&lt;T&gt;<\/code><\/td><td>\u5f15\u7528\u7c7b\u578b\u672c\u8eab\u53ef\u4ee5\u4e3a null\uff0c\u65e0\u9700\u518d\u7528 Nullable \u5305\u88c5<\/td><\/tr><tr><td>\u2705 \u914d\u5408 <code>switch<\/code>, <code>if<\/code>, <code>?.<\/code>, <code>??<\/code> \u7b49\u4f7f\u7528<\/td><td>\u6784\u5efa\u5065\u58ee\u7684\u7a7a\u503c\u5224\u65ad\u903b\u8f91<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">\u5341\u4e8c\u3001\u53ef\u7a7a\u5f15\u7528\u7c7b\u578b\uff08Nullable Reference Types\uff09[C# 8.0+]<\/h2>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p>\u4e0e\u672c\u6587\u8bb2\u7684 <strong>\u53ef\u7a7a\u503c\u7c7b\u578b<\/strong> \u4e0d\u540c\uff0cC# 8 \u5f15\u5165\u4e86 <strong>\u53ef\u7a7a\u5f15\u7528\u7c7b\u578b<\/strong>\uff0c\u7528\u4e8e\u589e\u5f3a\u4ee3\u7801\u7684 null \u5b89\u5168\u6027\u3002<\/p>\n<\/blockquote>\n\n\n\n<p>\u542f\u7528\u540e\uff0c\u53ef\u4ee5\u4f7f\u7528\uff1a<\/p>\n\n\n\n<pre class=\"wp-block-code\"><code>string? name = null;  \/\/ \u660e\u786e\u8868\u793a name \u53ef\u4e3a null<\/code><\/pre>\n\n\n\n<p>\u8be6\u60c5\u6587\u6863\u8bf7\u89c1\uff1a<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>\ud83d\udd17 <a href=\"https:\/\/learn.microsoft.com\/en-us\/dotnet\/csharp\/nullable-references\">Nullable Reference Types<\/a><\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p>\u66f4\u591a\u8be6\u7ec6\u5185\u5bb9\uff0c\u8bf7\u5173\u6ce8\u5176\u4ed6\u76f8\u5173\u6587\u7ae0\uff01<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u4e0b\u9762\u662f\u5bf9 C# \u53ef\u7a7a\u7c7b\u578b\uff08Nullable Types\uff09 \u7684\u5168\u9762\u4e13\u4e1a\u8bb2\u89e3\uff0c\u4fe1\u606f\u6765\u6e90\u4e8e Microsoft \u5b98\u65b9 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":3503,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[83],"tags":[],"class_list":["post-3502","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c-"],"_links":{"self":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/3502","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/comments?post=3502"}],"version-history":[{"count":1,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/3502\/revisions"}],"predecessor-version":[{"id":3504,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/posts\/3502\/revisions\/3504"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media\/3503"}],"wp:attachment":[{"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/media?parent=3502"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/categories?post=3502"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.laixuexila.com\/index.php\/wp-json\/wp\/v2\/tags?post=3502"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}