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POCan I create a generic method that takes a value type or a reference type but always returns a nullable type
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  1. POCan I create a generic method that takes a value type or a reference type but always returns a nullable type
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    copied!<p>This is my method. Note that I am returning the equivalent nullable type for the generic parameter <code>R</code>:</p> <pre><code> public static Nullable&lt;R&gt; GetValue&lt;T, R&gt;(this T a, Expression&lt;Func&lt;T, R&gt;&gt; expression) where T : Attribute where R : struct { if (a == null) return null; PropertyInfo p = GetProperty(expression); if (p == null) return null; return (R)p.GetValue(a, null); } </code></pre> <p>I can use it in a call to get the value of an attribute like this:</p> <pre><code>//I don't throw exceptions for invalid or missing calls //because I want to chain the calls together: int maximumLength4 = instance.GetProperty(x =&gt; x.ToString()) .GetAttribute&lt;StringLengthAttribute&gt;() .GetValue(x =&gt; x.MaximumLength) .GetValueOrDefault(50); </code></pre> <p>I'd like to use the same generic method with strings:</p> <pre><code>//I'd like to use the GetValue generic method with strings as well as integers string erroMessage = instance.GetProperty(x =&gt; x.ToString()) .GetAttribute&lt;StringLengthAttribute&gt;() .GetValue(x =&gt; x.ErrorMessage); </code></pre> <p>but it won't compile:</p> <blockquote> <p>The type 'R' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'</p> <p>Cannot implicitly convert type 'string?' to 'string'</p> </blockquote> <p>Is there any trick I can use to get the same method signature here and yet get generics to infer the return type as one that can be null?</p> <p>This is some test code to show that it works for integer values:</p> <pre><code>//[StringLength(256)] //public string Name { get; set; } PropertyInfo info = ReflectionAPI.GetProperty&lt;Organisation, String&gt;(x =&gt; x.Name);//not null StringLengthAttribute attr = info.GetAttribute&lt;StringLengthAttribute&gt;();//not null int? maximumLength = attr.GetValue(x =&gt; x.MaximumLength);//256 int? minimumLength = attr.GetValue(x =&gt; x.MinimumLength);//0 PropertyInfo info2 = ReflectionAPI.GetProperty&lt;Organisation, int&gt;(x =&gt; x.ID);//not null StringLengthAttribute attr2 = info2.GetAttribute&lt;StringLengthAttribute&gt;();//null because ID doesn't have the attribute int? maximumLength2 = attr2.GetValue(x =&gt; x.MaximumLength);//null int? minimumLength2 = attr2.GetValue(x =&gt; x.MinimumLength);//null //I can use the GetProperty extension method on an instance Organisation instance = (Organisation)null; PropertyInfo info3 = instance.GetProperty(x =&gt; x.ToString());//null because its a method call not a property StringLengthAttribute attr3 = info3.GetAttribute&lt;StringLengthAttribute&gt;();//null int? maximumLength3 = attr3.GetValue(x =&gt; x.MaximumLength);//null int? minimumLength3 = attr3.GetValue(x =&gt; x.MinimumLength);//null </code></pre> <p>And this is the rest of my <code>ReflectionAPI</code>:</p> <pre><code>public static class ReflectionAPI { public static Nullable&lt;R&gt; GetValue&lt;T, R&gt;(this T a, Expression&lt;Func&lt;T, R&gt;&gt; expression) where T : Attribute { if (a == null) return null; PropertyInfo p = GetProperty(expression); if (p == null) return null; return (R)p.GetValue(a, null); } public static T GetAttribute&lt;T&gt;(this PropertyInfo p) where T : Attribute { if (p == null) return null; return p.GetCustomAttributes(false).OfType&lt;T&gt;().LastOrDefault(); } public static PropertyInfo GetProperty&lt;T, R&gt;(Expression&lt;Func&lt;T, R&gt;&gt; expression) { if (expression == null) return null; MemberExpression memberExpression = expression.Body as MemberExpression; if (memberExpression == null) return null; return memberExpression.Member as PropertyInfo; } } </code></pre>
 

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