blocks|key|618149|text|关于|type|unstyled|depth|inlineStyleRanges|entityRanges|data|618150|string+y+=+(Session["key"]+??+"none").ToString();|code-block|syntax|javascript|618151|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

What about 

<pre><code>string y = (Session["key"] ?? "none").ToString();
</code></pre>

blocks|key|618171|text|创建辅助函数|type|unstyled|depth|inlineStyleRanges|entityRanges|data|618172|public+static+String+GetValue(+string+key,+string+default+)
{
++++if+(+Session[+key+]+==+null+)+{+return+default;+}
++++return+Session[+key+].toString();
}


string+y+=+GetValue(+'key',+'none'+);|code-block|syntax|javascript|618173|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

create an auxiliary function

<pre><code>public static String GetValue( string key, string default )
{
 if ( Session[ key ] == null ) { return default; }
 return Session[ key ].toString();
}


string y = GetValue( 'key', 'none' );
</code></pre>

blocks|key|618231|text|如果它始终是一个string，您可以强制转换：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|618232|string+y+=+(string)Session["key"]+??+"none";|code-block|syntax|javascript|618233|这样做的好处是，如果有人在Session["key"]中填充了int或其他东西，则可以抱怨，而不是隐藏错误。;)|618234|entityMap^0|8|6|0|0|D|E|V|3|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

If it will always be a <code>string</code>, you can cast:

<pre><code>string y = (string)Session["key"] ?? "none";
</code></pre>

This has the advantage of complaining instead of hiding the mistake if someone stuffs an <code>int</code> or something in <code>Session["key"]</code>. ;)

blocks|key|402749|text|所有建议的解决方案都很好，并且回答了这个问题；所以这里只是稍微扩展一下。目前，大多数答案只涉及null验证和字符串类型。您可以扩展StateBag对象以包含一个泛型GetValueOrDefault方法，类似于Jon+Skeet发布的答案。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|402750|一个简单的泛型扩展方法，它接受一个字符串作为键，然后类型检查会话对象。如果对象为空或不是同一类型，则返回默认值，否则返回强类型的会话值。|402751|像这样的东西|402752|///+<summary>
///+Gets+a+value+from+the+current+session,+if+the+type+is+correct+and+present
///+</summary>
///+<param+name="key">The+session+key</param>
///+<param+name="defaultValue">The+default+value</param>
///+<returns>Returns+a+strongly+typed+session+object,+or+default+value</returns>
public+static+T+GetValueOrDefault<T>(this+HttpSessionState+source,+string+key,+T+defaultValue)
{
++++//+check+if+the+session+object+exists,+and+is+of+the+correct+type
++++object+value+=+source[key]
++++if+(value+==+null+%7C%7C+!(value+is+T))
++++{
++++++++return+defaultValue;
++++}

++++//+return+the+session+object
++++return+(T)value;
}|code-block|syntax|javascript|402753|entityMap^0|1T|8|2A|H|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|W|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|X|8|@]|D|@]|E|$M|N]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

All of the suggested solutions are good, and answer the question; so this is just to extend on it slightly. Currently the majority of answers only deal with null validation and string types. You could extend the <code>StateBag</code> object to include a generic <code>GetValueOrDefault</code> method, similar to the answer posted by Jon Skeet.

A simple generic extension method that accepts a string as a key, and then type checks the session object. If the object is null or not the same type, the default is returned, otherwise the session value is returned strongly typed.

Something like this

<pre><code>/// &lt;summary&gt;
/// Gets a value from the current session, if the type is correct and present
/// &lt;/summary&gt;
/// &lt;param name="key"&gt;The session key&lt;/param&gt;
/// &lt;param name="defaultValue"&gt;The default value&lt;/param&gt;
/// &lt;returns&gt;Returns a strongly typed session object, or default value&lt;/returns&gt;
public static T GetValueOrDefault&lt;T&gt;(this HttpSessionState source, string key, T defaultValue)
{
 // check if the session object exists, and is of the correct type
 object value = source[key]
 if (value == null || !(value is T))
 {
 return defaultValue;
 }

 // return the session object
 return (T)value;
}
</code></pre>

blocks|key|402818|text|您还可以使用as，如果转换失败，它会生成null：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|402819|Session["key"]+as+string+??+"none"|code-block|syntax|javascript|402820|这将返回"none"，即使有人在Session["key"]中填充了一个int。|402821|entityMap|0|LINK|mutability|MUTABLE|url|http://msdn.microsoft.com/en-us/library/cscsdfbt(v=vs.71).aspx)^0|6|2|K|4|6|2|0|0|0|4|6|G|E|10|3|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@$9|Z|A|10|1|11]]|E|$]]|$1|F|3|G|5|H|7|12|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

You could also use <a href="http://msdn.microsoft.com/en-us/library/cscsdfbt(v=vs.71).aspx" rel="nofollow noreferrer"><code>as</code></a>, which yields <code>null</code> if the conversion fails:

<pre><code>Session["key"] as string ?? "none"
</code></pre>

This would return <code>"none"</code> even if someone stuffed an <code>int</code> in <code>Session["key"]</code>.

blocks|key|618538|text|对于one+off，我的首选是使用安全的强制转换到字符串，以防与键一起存储的对象不是一个。使用ToString()可能不会得到您想要的结果。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|618539|var+y+=+Session["key"]+as+string+??+"none";|code-block|syntax|javascript|618540|正如@Jon+Skeet所说，如果您发现自己经常这样做，那么可以使用扩展方法，或者更好的方法，可以将扩展方法与强类型的SessionWrapper类结合使用。即使没有扩展方法，强类型包装器也可能是一个好主意。|618541|public+class+SessionWrapper
{
++++private+HttpSessionBase+Session+{+get;+set;+}

++++public+SessionWrapper(+HttpSessionBase+session+)
++++{
++++++++Session+=+session;
++++}

++++public+SessionWrapper()+:+this(+HttpContext.Current.Session+)+{+}

++++public+string+Key
++++{
+++++++++get+{+return+Session["key"]+as+string+??+"none";
++++}

++++public+int+MaxAllowed
++++{
+++++++++get+{+return+Session["maxAllowed"]+as+int?+??+10+}
++++}
}|618542|用作|618543|+var+session+=+new+SessionWrapper(Session);

+string+key+=+session.Key;
+int+maxAllowed+=+session.maxAllowed;|618544|entityMap^0|1B|A|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|T|$]]

My preference, for a one off, would be to use a safe cast to string in case the object stored with the key isn't one. Using <code>ToString()</code> may not have the results you want.

<pre><code>var y = Session["key"] as string ?? "none";
</code></pre>

As @Jon Skeet says, if you find yourself doing this a lot an extension method or, better, yet maybe an extension method in conjunction with a strongly typed SessionWrapper class. Even without the extension method, the strongly typed wrapper might be a good idea.

<pre><code>public class SessionWrapper
{
 private HttpSessionBase Session { get; set; }

 public SessionWrapper( HttpSessionBase session )
 {
 Session = session;
 }

 public SessionWrapper() : this( HttpContext.Current.Session ) { }

 public string Key
 {
 get { return Session["key"] as string ?? "none";
 }

 public int MaxAllowed
 {
 get { return Session["maxAllowed"] as int? ?? 10 }
 }
}
</code></pre>

Used as

<pre><code> var session = new SessionWrapper(Session);

 string key = session.Key;
 int maxAllowed = session.maxAllowed;
</code></pre>

blocks|key|618599|text|我们使用一个名为NullOr的方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|618600|用法|618601|//+Call+ToString()+if+it’s+not+null,+otherwise+return+null
var+str+=+myObj.NullOr(obj+=>+obj.ToString());

//+Supply+default+value+for+when+it’s+null
var+str+=+myObj.NullOr(obj+=>+obj.ToString())+??+"none";

//+Works+with+nullable+return+values,+too+—
//+this+is+properly+typed+as+“int?”+(nullable+int)
//+even+if+“Count”+is+just+int
var+count+=+myCollection.NullOr(coll+=>+coll.Count);

//+Works+with+nullable+input+types,+too
int?+unsure+=+47;
var+sure+=+unsure.NullOr(i+=>+i.ToString());|code-block|syntax|javascript|618602|来源|618603|///+<summary>Provides+a+function+delegate+that+accepts+only+value+types+as+return+types.</summary>
///+<remarks>This+type+was+introduced+to+make+<see+cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncStruct{TInput,TResult})"/>
///+work+without+clashing+with+<see+cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncClass{TInput,TResult})"/>.</remarks>
public+delegate+TResult+FuncStruct<in+TInput,+TResult>(TInput+input)+where+TResult+:+struct;

///+<summary>Provides+a+function+delegate+that+accepts+only+reference+types+as+return+types.</summary>
///+<remarks>This+type+was+introduced+to+make+<see+cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncClass{TInput,TResult})"/>
///+work+without+clashing+with+<see+cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncStruct{TInput,TResult})"/>.</remarks>
public+delegate+TResult+FuncClass<in+TInput,+TResult>(TInput+input)+where+TResult+:+class;

///+<summary>Provides+extension+methods+that+apply+to+all+types.</summary>
public+static+class+ObjectExtensions
{
++++///+<summary>Returns+null+if+the+input+is+null,+otherwise+the+result+of+the+specified+lambda+when+applied+to+the+input.</summary>
++++///+<typeparam+name="TInput">Type+of+the+input+value.</typeparam>
++++///+<typeparam+name="TResult">Type+of+the+result+from+the+lambda.</typeparam>
++++///+<param+name="input">Input+value+to+check+for+null.</param>
++++///+<param+name="lambda">Function+to+apply+the+input+value+to+if+it+is+not+null.</param>
++++public+static+TResult+NullOr<TInput,+TResult>(this+TInput+input,+FuncClass<TInput,+TResult>+lambda)+where+TResult+:+class
++++{
++++++++return+input+==+null+?+null+:+lambda(input);
++++}

++++///+<summary>Returns+null+if+the+input+is+null,+otherwise+the+result+of+the+specified+lambda+when+applied+to+the+input.</summary>
++++///+<typeparam+name="TInput">Type+of+the+input+value.</typeparam>
++++///+<typeparam+name="TResult">Type+of+the+result+from+the+lambda.</typeparam>
++++///+<param+name="input">Input+value+to+check+for+null.</param>
++++///+<param+name="lambda">Function+to+apply+the+input+value+to+if+it+is+not+null.</param>
++++public+static+TResult?+NullOr<TInput,+TResult>(this+TInput+input,+Func<TInput,+TResult?>+lambda)+where+TResult+:+struct
++++{
++++++++return+input+==+null+?+null+:+lambda(input);
++++}

++++///+<summary>Returns+null+if+the+input+is+null,+otherwise+the+result+of+the+specified+lambda+when+applied+to+the+input.</summary>
++++///+<typeparam+name="TInput">Type+of+the+input+value.</typeparam>
++++///+<typeparam+name="TResult">Type+of+the+result+from+the+lambda.</typeparam>
++++///+<param+name="input">Input+value+to+check+for+null.</param>
++++///+<param+name="lambda">Function+to+apply+the+input+value+to+if+it+is+not+null.</param>
++++public+static+TResult?+NullOr<TInput,+TResult>(this+TInput+input,+FuncStruct<TInput,+TResult>+lambda)+where+TResult+:+struct
++++{
++++++++return+input+==+null+?+null+:+lambda(input).Nullable();
++++}
}|618604|entityMap^0|8|6|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

We use a method called <code>NullOr</code>.

<h2>Usage</h2>

<pre><code>// Call ToString() if it’s not null, otherwise return null
var str = myObj.NullOr(obj =&gt; obj.ToString());

// Supply default value for when it’s null
var str = myObj.NullOr(obj =&gt; obj.ToString()) ?? "none";

// Works with nullable return values, too —
// this is properly typed as “int?” (nullable int)
// even if “Count” is just int
var count = myCollection.NullOr(coll =&gt; coll.Count);

// Works with nullable input types, too
int? unsure = 47;
var sure = unsure.NullOr(i =&gt; i.ToString());
</code></pre>

<h2>Source</h2>

<pre><code>/// &lt;summary&gt;Provides a function delegate that accepts only value types as return types.&lt;/summary&gt;
/// &lt;remarks&gt;This type was introduced to make &lt;see cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncStruct{TInput,TResult})"/&gt;
/// work without clashing with &lt;see cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncClass{TInput,TResult})"/&gt;.&lt;/remarks&gt;
public delegate TResult FuncStruct&lt;in TInput, TResult&gt;(TInput input) where TResult : struct;

/// &lt;summary&gt;Provides a function delegate that accepts only reference types as return types.&lt;/summary&gt;
/// &lt;remarks&gt;This type was introduced to make &lt;see cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncClass{TInput,TResult})"/&gt;
/// work without clashing with &lt;see cref="ObjectExtensions.NullOr{TInput,TResult}(TInput,FuncStruct{TInput,TResult})"/&gt;.&lt;/remarks&gt;
public delegate TResult FuncClass&lt;in TInput, TResult&gt;(TInput input) where TResult : class;

/// &lt;summary&gt;Provides extension methods that apply to all types.&lt;/summary&gt;
public static class ObjectExtensions
{
 /// &lt;summary&gt;Returns null if the input is null, otherwise the result of the specified lambda when applied to the input.&lt;/summary&gt;
 /// &lt;typeparam name="TInput"&gt;Type of the input value.&lt;/typeparam&gt;
 /// &lt;typeparam name="TResult"&gt;Type of the result from the lambda.&lt;/typeparam&gt;
 /// &lt;param name="input"&gt;Input value to check for null.&lt;/param&gt;
 /// &lt;param name="lambda"&gt;Function to apply the input value to if it is not null.&lt;/param&gt;
 public static TResult NullOr&lt;TInput, TResult&gt;(this TInput input, FuncClass&lt;TInput, TResult&gt; lambda) where TResult : class
 {
 return input == null ? null : lambda(input);
 }

 /// &lt;summary&gt;Returns null if the input is null, otherwise the result of the specified lambda when applied to the input.&lt;/summary&gt;
 /// &lt;typeparam name="TInput"&gt;Type of the input value.&lt;/typeparam&gt;
 /// &lt;typeparam name="TResult"&gt;Type of the result from the lambda.&lt;/typeparam&gt;
 /// &lt;param name="input"&gt;Input value to check for null.&lt;/param&gt;
 /// &lt;param name="lambda"&gt;Function to apply the input value to if it is not null.&lt;/param&gt;
 public static TResult? NullOr&lt;TInput, TResult&gt;(this TInput input, Func&lt;TInput, TResult?&gt; lambda) where TResult : struct
 {
 return input == null ? null : lambda(input);
 }

 /// &lt;summary&gt;Returns null if the input is null, otherwise the result of the specified lambda when applied to the input.&lt;/summary&gt;
 /// &lt;typeparam name="TInput"&gt;Type of the input value.&lt;/typeparam&gt;
 /// &lt;typeparam name="TResult"&gt;Type of the result from the lambda.&lt;/typeparam&gt;
 /// &lt;param name="input"&gt;Input value to check for null.&lt;/param&gt;
 /// &lt;param name="lambda"&gt;Function to apply the input value to if it is not null.&lt;/param&gt;
 public static TResult? NullOr&lt;TInput, TResult&gt;(this TInput input, FuncStruct&lt;TInput, TResult&gt; lambda) where TResult : struct
 {
 return input == null ? null : lambda(input).Nullable();
 }
}
</code></pre>

blocks|key|402987|text|这是我的小类型安全“猫王运算符”，适用于不支持?的.NET版本。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|402988|public+class+IsNull
{
++++public+static+O+Substitute<I,O>(I+obj,+Func<I,O>+fn,+O+nullValue=default(O))
++++{
++++++++if+(obj+==+null)
++++++++++++return+nullValue;
++++++++else
++++++++++++return+fn(obj);
++++}
}|code-block|syntax|javascript|402989|第一个参数是被测试的对象。二是功能。第三个是空值。因此，对于你的情况：|402990|IsNull.Substitute(Session["key"],s=>s.ToString(),"none");|402991|对于可以为空的类型，它也非常有用。例如：|402992|decimal?+v;
...
IsNull.Substitute(v,v.Value,0);
....|402993|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

This is my little type safe "Elvis operator" for versions of .NET that do not support ?.

<pre><code>public class IsNull
{
 public static O Substitute&lt;I,O&gt;(I obj, Func&lt;I,O&gt; fn, O nullValue=default(O))
 {
 if (obj == null)
 return nullValue;
 else
 return fn(obj);
 }
}
</code></pre>

First argument is the tested object. Second is the function. And third is the null value. So for your case:

<pre><code>IsNull.Substitute(Session["key"],s=&gt;s.ToString(),"none");
</code></pre>

It is very useful for nullable types too. For example:

<pre><code>decimal? v;
...
IsNull.Substitute(v,v.Value,0);
....
</code></pre>

I love the null-coalescing operator because it makes it easy to assign a default value for nullable types.

<pre><code> int y = x ?? -1;
</code></pre>

That's great, except if I need to do something simple with <code>x</code>. For instance, if I want to check <code>Session</code>, then I usually end up having to write something more verbose.

I wish I could do this:

<pre><code>string y = Session["key"].ToString() ?? "none";
</code></pre>

But you can't because the <code>.ToString()</code> gets called before the null check so it fails if <code>Session["key"]</code> is null. I end up doing this:

<pre><code>string y = Session["key"] == null ? "none" : Session["key"].ToString();
</code></pre>

It works and is better, in my opinion, than the three-line alternative:

<pre><code>string y = "none";
if (Session["key"] != null)
 y = Session["key"].ToString();
</code></pre>

Even though that works I am still curious if there is a better way. It seems no matter what I always have to reference <code>Session["key"]</code> twice; once for the check, and again for the assignment. Any ideas?

What is the proper way to check for null values?

我喜欢null-coalescing运算符，因为它可以很容易地为可为null的类型分配默认值。 int y = x ?? -1;这很好，除非我需要用x做一些简单的事情。例如，如果我想检查Session，那么我通常不得不写一些更冗长的东西。我希望我能这样做：string y = Session["key"].ToStri...

问检查空值的正确方法是什么？
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