问题
In C#,
Is there a way to turn an automatic property into a lazy loaded automatic property with a specified default value?
Essentially, I am trying to turn this...
private string _SomeVariable
public string SomeVariable
{
get
{
if(_SomeVariable == null)
{
_SomeVariable = SomeClass.IOnlyWantToCallYouOnce();
}
return _SomeVariable;
}
}
into something different, where I can specify the default and it handles the rest automatically...
[SetUsing(SomeClass.IOnlyWantToCallYouOnce())]
public string SomeVariable {get; private set;}
回答1:
No there is not. Auto-implemented properties only function to implement the most basic of properties: backing field with getter and setter. It doesn't support this type of customization.
However you can use the 4.0 Lazy<T> type to create this pattern
private Lazy<string> _someVariable =new Lazy<string>(SomeClass.IOnlyWantToCallYouOnce);
public string SomeVariable => _someVariable.Value;
This code will lazily calculate the value of _someVariable the first time the Value expression is called. It will only be calculated once and will cache the value for future uses of the Value property
回答2:
Probably the most concise you can get is to use the null-coalescing operator:
get { return _SomeVariable ?? (_SomeVariable = SomeClass.IOnlyWantToCallYouOnce()); }
回答3:
There is a new feature in C#6 called Expression Bodied Auto-Properties, which allows you to write it a bit cleaner:
public class SomeClass
{
private Lazy<string> _someVariable = new Lazy<string>(SomeClass.IOnlyWantToCallYouOnce);
public string SomeVariable
{
get { return _someVariable.Value; }
}
}
Can now be written as:
public class SomeClass
{
private Lazy<string> _someVariable = new Lazy<string>(SomeClass.IOnlyWantToCallYouOnce);
public string SomeVariable => _someVariable.Value;
}
回答4:
Not like that, parameters for attributes must be constant in value, you cannot call code (Even static code).
You may however be able to implement something with PostSharp's Aspects.
Check them out:
PostSharp
回答5:
Here's my implementation of a solve to your problem. Basically the idea is a property that will be set by a function at first access and subsequent accesses will yield the same return value as the first.
public class LazyProperty<T>
{
bool _initialized = false;
T _result;
public T Value(Func<T> fn)
{
if (!_initialized)
{
_result = fn();
_initialized = true;
}
return _result;
}
}
Then to use:
LazyProperty<Color> _eyeColor = new LazyProperty<Color>();
public Color EyeColor
{
get
{
return _eyeColor.Value(() => SomeCPUHungryMethod());
}
}
There is of course the overhead of passing the function pointer around, but it does the job for me and I don't notice too much overhead compared to running the method over and over again.
回答6:
I'm a big fan of this idea, and would like to offer up the following C# snippet which I called proplazy.snippet.(you can either import this or paste it into the standard folder which you can get from the Snippet Manager)
Here's a sample of its output:
private Lazy<int> myProperty = new Lazy<int>(()=>1);
public int MyProperty { get { return myProperty.Value; } }
Here's the snippet file contents: (save as proplazy.snippet)
<?xml version="1.0" encoding="utf-8" ?>
<CodeSnippets xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
<CodeSnippet Format="1.0.0">
<Header>
<Title>proplazy</Title>
<Shortcut>proplazy</Shortcut>
<Description>Code snippet for property and backing field</Description>
<Author>Microsoft Corporation</Author>
<SnippetTypes>
<SnippetType>Expansion</SnippetType>
</SnippetTypes>
</Header>
<Snippet>
<Declarations>
<Literal>
<ID>type</ID>
<ToolTip>Property type</ToolTip>
<Default>int</Default>
</Literal>
<Literal>
<ID>field</ID>
<ToolTip>The variable backing this property</ToolTip>
<Default>myVar</Default>
</Literal>
<Literal>
<ID>func</ID>
<ToolTip>The function providing the lazy value</ToolTip>
</Literal>
<Literal>
<ID>property</ID>
<ToolTip>Property name</ToolTip>
<Default>MyProperty</Default>
</Literal>
</Declarations>
<Code Language="csharp"><![CDATA[private Lazy<$type$> $field$ = new Lazy<$type$>($func$);
public $type$ $property$ { get{ return $field$.Value; } }
$end$]]>
</Code>
</Snippet>
</CodeSnippet>
</CodeSnippets>
回答7:
I don't think this is possible with pure C#. But you could do it using an IL rewriter like PostSharp. For example it allows you to add handlers before and after functions depending on attributes.
回答8:
I did it like this:
public static class LazyCachableGetter
{
private static ConditionalWeakTable<object, IDictionary<string, object>> Instances = new ConditionalWeakTable<object, IDictionary<string, object>>();
public static R LazyValue<T, R>(this T obj, Func<R> factory, [CallerMemberName] string prop = "")
{
R result = default(R);
if (!ReferenceEquals(obj, null))
{
if (!Instances.TryGetValue(obj, out var cache))
{
cache = new ConcurrentDictionary<string, object>();
Instances.Add(obj, cache);
}
if (!cache.TryGetValue(prop, out var cached))
{
cache[prop] = (result = factory());
}
else
{
result = (R)cached;
}
}
return result;
}
}
and later you can use it like
public virtual bool SomeProperty => this.LazyValue(() =>
{
return true;
});
回答9:
https://github.com/bcuff/AutoLazy uses Fody to give you something like this
public class MyClass
{
// This would work as a method, e.g. GetSettings(), as well.
[Lazy]
public static Settings Settings
{
get
{
using (var fs = File.Open("settings.xml", FileMode.Open))
{
var serializer = new XmlSerializer(typeof(Settings));
return (Settings)serializer.Deserialize(fs);
}
}
}
[Lazy]
public static Settings GetSettingsFile(string fileName)
{
using (var fs = File.Open(fileName, FileMode.Open))
{
var serializer = new XmlSerializer(typeof(Settings));
return (Settings)serializer.Deserialize(fs);
}
}
}
回答10:
[Serializable]
public class RaporImza
{
private readonly Func<ReportConfig> _getReportLayout;
public RaporImza(Func<ReportConfig> getReportLayout)
{
_getReportLayout = getReportLayout;
}
private ReportConfig _getReportLayoutResult;
public ReportConfig GetReportLayoutResult => _getReportLayoutResult ?? (_getReportLayoutResult = _getReportLayout());
public string ImzaAtanKisiAdi => GetReportLayoutResult.ReportSignatureName;
public string ImzaAtanKisiUnvani => GetReportLayoutResult.ReportSignatureTitle;
public byte[] Imza => GetReportLayoutResult.ReportSignature;
}
and i call like bellow
result.RaporBilgisi = new ExchangeProgramPersonAllDataModel.RaporImza(() => _reportConfigService.GetReportLayout(documentTypeId));
回答11:
If you use a constructor during lazy initialization, following extensions may be helpful too
public static partial class New
{
public static T Lazy<T>(ref T o) where T : class, new() => o ?? (o = new T());
public static T Lazy<T>(ref T o, params object[] args) where T : class, new() =>
o ?? (o = (T) Activator.CreateInstance(typeof(T), args));
}
Usage
private Dictionary<string, object> _cache;
public Dictionary<string, object> Cache => New.Lazy(ref _cache);
/* _cache ?? (_cache = new Dictionary<string, object>()); */
来源：https://stackoverflow.com/questions/4036812/c-sharp-lazy-loaded-automatic-properties