nullable

问题 Is there a converter in .NET 4.0 that supports conversions between nullable types to shorten instructions like: bool? nullableBool = GetSomething(); byte? nbyte = nullableBool.HasValue ? (byte?)Convert.ToByte(nullableBool.Value) : null; 回答1: Not that I am aware of. You could just write a helper method like this: public Nullable<TTarget> NullableConvert<TSource, TTarget>( Nullable<TSource> source, Func<TSource, TTarget> converter) where TTarget: struct where TSource: struct { return source

问题 how to create setter and getter Properties for nullable datetime. for example: private DateTime mTimeStamp; public DateTime TimeStamp { get { return mTimeStamp; } set { mTimeStamp = value; } } Does nullable attributes support setter and getter or have i to declare it public? private DateTime? mTimeStamp; public DateTime TimeStamp { } 回答1: You can just do this instead: public DateTime? TimeStamp { get; set; } If you were having trouble with the compiler it's probably because you only changed

问题 I try to set a Nullable<> property dynamicly. I Get my property ex : PropertyInfo property = class.GetProperty("PropertyName"); // My property is Nullable<> at this time So the type could be a string or int I want to set my property by reflection like property.SetValue(class,"1256",null); It's not working when my property is a Nullable<> Generic. So i try to find a way to set my property. To know the type of my nullable<> property i execute Nullable.GetUnderlyingType(property.PropertyType)

问题 I have a simple method that returns a Nullable Int32 from a DataReader rather than the built in GetInt32. I am calling this method many many times and have one situation where any time I can shave off of it would be beneficial. Can anyone suggest any alternative and faster ways of getting a nullable Int32 out of the DataReader? private Int32? GetNullableInt32(SqlDataReader dataReader, int fieldIndex) { return !dataReader.IsDBNull(fieldIndex) ? dataReader.GetInt32(fieldIndex) : (Int32?)null; }

问题 I have a simple method that returns a Nullable Int32 from a DataReader rather than the built in GetInt32. I am calling this method many many times and have one situation where any time I can shave off of it would be beneficial. Can anyone suggest any alternative and faster ways of getting a nullable Int32 out of the DataReader? private Int32? GetNullableInt32(SqlDataReader dataReader, int fieldIndex) { return !dataReader.IsDBNull(fieldIndex) ? dataReader.GetInt32(fieldIndex) : (Int32?)null; }

问题 Consider the following code: DateTime t = DateTime.Today; bool isGreater = t > null; With Visual Studio 2010 (C# 4, .NET 4.0), I get the following warning: warning CS0458: The result of the expression is always 'null' of type 'bool?' This is incorrect; the result is always false (of type bool ): Now, the struct DateTime overloads the > (greater than) operator. Any non-nullable struct (like DateTime) is implicitly convertible to the corresponding Nullable<> type. The above expression is

问题 C# 8.0 introduces nullable reference types. Here's a simple class with a nullable property: public class Foo { public String? Bar { get; set; } } Is there a way to check a class property uses a nullable reference type via reflection? 回答1: This appears to work, at least on the types I've tested it with. You need to pass the PropertyInfo for the property you're interested in, and also the Type which that property is defined on ( not a derived or parent type - it has to be the exact type):

问题 There is a lot of syntax sugar with Nullable<T> like those: int? parsed to Nullable<int> int? x = null if (x != null) // Parsed to if (x.HasValue) x = 56; // Parsed to x.Value = 56; And more. Why if condition with Nullable doesn't work? if (x) {} It gets Complier error saying can't convert Nullable<bool> to bool . Why it's not being parsed to if (x.HasValue && x.Value == true) or something similar? It's the most obvious usage for Nullable<bool> 回答1: It's the most obvious usage for Nullable

问题 Often, I can see a code constructs like following: if(a == null || b == null || c == null){ //... } I wonder if there is any widely used library (Google, Apache, etc.) to check against nullity for multiple objects at once, e.g.: if(anyIsNull(a, b, c)){ //... } or if(allAreNulls(a, b, c)){ //... } UPDATE: I perfectly know how to write it by myself I know it can be the result of the poor program structure but it's not a case here Let's make it more challenging and replace original example with

问题 How do i check if a Type is a nullable enum in C# something like Type t = GetMyType(); bool isEnum = t.IsEnum; //Type member bool isNullableEnum = t.IsNullableEnum(); How to implement this extension method? 回答1: public static bool IsNullableEnum(this Type t) { Type u = Nullable.GetUnderlyingType(t); return (u != null) && u.IsEnum; } 回答2: EDIT: I'm going to leave this answer up as it will work, and it demonstrates a few calls that readers may not otherwise know about. However, Luke's answer is