C# 将空值转换为字符串的函数

A function to convert null to string

提问by James Andrew Smith

I want to create a function to convert any null value e.g. from a database to an empty string. I know there are methods such as if value != null ?? value : String.Emptybut is there a way to pass nullto a method e.g.

我想创建一个函数来转换任何空值，例如从数据库到空字符串。我知道有一些方法，例如 ifvalue != null ?? value : String.Empty但是有没有办法传递null给一个方法，例如

public string nullToString(string? value)
{
    if(value == null) return empty;
    return value
}

But I am not sure on the parameter syntax to do this..

但我不确定要执行此操作的参数语法。

I tried the above but it says not a nullable type.

我尝试了上面的方法，但它说不是可以为空的类型。

Thanks in advance.

提前致谢。

采纳答案by Gareth Wilson

static string NullToString( object Value )
{

    // Value.ToString() allows for Value being DBNull, but will also convert int, double, etc.
    return Value == null ? "" : Value.ToString();

    // If this is not what you want then this form may suit you better, handles 'Null' and DBNull otherwise tries a straight cast
    // which will throw if Value isn't actually a string object.
    //return Value == null || Value == DBNull.Value ? "" : (string)Value;


}

回答by Shyju

public string nullToString(string value)
{
    return value == null ?string.Empty: value;   
}

回答by cadrell0

You can just use the null coalesce operator.

您可以只使用空合并运算符。

string result = value ?? "";

回答by Icarus

When you get a NULL value from a database, the value returned is DBNull.Value on which case, you can simply call .ToString()and it will return ""

当您从数据库中获取 NULL 值时，返回的值为 DBNull.Value 在这种情况下，您只需调用.ToString()它就会返回""

Example:

例子：

 reader["Column"].ToString() 

Gets you ""if the value returned is DBNull.Value

让你""如果返回的值是DBNull.Value

If the scenario is not always a database, then I'd go for an Extension method:

如果场景并不总是一个数据库，那么我会使用 Extension 方法：

public static class Extensions
{

    public static string EmptyIfNull(this object value)
    {
        if (value == null)
            return "";
        return value.ToString();
    }
}

Usage:

用法：

string someVar = null; 
someVar.EmptyIfNull();

回答by Rafiqzzaman Liton

 public string ToString(this string value)
        {
            if (value == null)
            {
                value = string.Empty;
            }               
            else
            {
                return value.Trim();
            }
        }

回答by live-love

Convert.ToString(object)converts to string. If the object is null, Convert.ToStringconverts it to an empty string.

Convert.ToString(object)转换为字符串。如果对象是null，Convert.ToString则将其转换为空字符串。

Calling .ToString()on an object with a null value throws a System.NullReferenceException.

调用.ToString()具有空值的对象会抛出System.NullReferenceException.

EDIT:

编辑：

Two exceptions to the rules:

规则的两个例外：

1) ConvertToString(string)on a null string will always return null.

1)ConvertToString(string)在空字符串上将始终返回空值。

2) ToString(Nullable<T>)on a null value will return "" .

2)ToString(Nullable<T>)在空值上将返回 "" 。

Code Sample:

代码示例：

// 1) Objects:

object obj = null;

//string valX1 = obj.ToString();           // throws System.NullReferenceException !!!
string val1 = Convert.ToString(obj);    

Console.WriteLine(val1 == ""); // True
Console.WriteLine(val1 == null); // False


// 2) Strings

String str = null;
//string valX2 = str.ToString();    // throws System.NullReferenceException !!!
string val2 = Convert.ToString(str); 

Console.WriteLine(val2 == ""); // False
Console.WriteLine(val2 == null); // True            


// 3) Nullable types:

long? num = null;
string val3 = num.ToString();  // ok, no error

Console.WriteLine(num == null); // True
Console.WriteLine(val3 == "");  // True
Console.WriteLine(val3 == null); // False 

val3 = Convert.ToString(num);  

Console.WriteLine(num == null);  // True
Console.WriteLine(val3 == "");  // True
Console.WriteLine(val3 == null); // False

回答by Stephen Turner

You can use Convert.ToString((object)value). You need to cast your value to an object first, otherwise the conversion will result in a null.

您可以使用Convert.ToString((object)value). 您需要先将您的值转换为对象，否则转换将导致空值。

using System;

public class Program
{
    public static void Main()
    {
        string format = "    Convert.ToString({0,-20}) == null? {1,-5},  == empty? {2,-5}";
        object nullObject = null;
        string nullString = null;

        string convertedString = Convert.ToString(nullObject);
        Console.WriteLine(format, "nullObject", convertedString == null, convertedString == "");

        convertedString = Convert.ToString(nullString);
        Console.WriteLine(format, "nullString", convertedString == null, convertedString == "");

        convertedString = Convert.ToString((object)nullString);
        Console.WriteLine(format, "(object)nullString", convertedString == null, convertedString == "");

    }
}

Gives:

给出：

Convert.ToString(nullObject          ) == null? False,  == empty? True 
Convert.ToString(nullString          ) == null? True ,  == empty? False
Convert.ToString((object)nullString  ) == null? False,  == empty? True

If you pass a System.DBNull.Value to Convert.ToString() it will be converted to an empty string too.

如果您将 System.DBNull.Value 传递给 Convert.ToString()，它也会被转换为空字符串。

回答by Trojaner

Its possible to make this even shorter with C# 6:

使用 C# 6 可以使这个更短：

public string NullToString(string Value)
{
    return value?.ToString() ?? "";
}

回答by Rogerio Azevedo

Its an old topic, but there is a "elegant" way to do that...

这是一个古老的话题，但有一种“优雅”的方式来做到这一点......

static string NullToString( object Value )
{       
    return Value = Value ?? string.Empty;    
}

回答by David Orbelian

It is possible to use the "?." (null conditional member access) with the "??" (null-coalescing operator) like this:

可以使用“？” （空条件成员访问）与“??” （空合并运算符）像这样：

public string EmptyIfNull(object value)
{
    return value?.ToString() ?? string.Empty;
}

This method can also be written as an extension method for an object:

这个方法也可以写成一个对象的扩展方法：

public static class ObjectExtensions
{
    public static string EmptyIfNull(this object value)
    {
        return value?.ToString() ?? string.Empty;
    }
}

And you can write same methods using "=>" (lambda operator):

您可以使用“=>”（lambda 运算符）编写相同的方法：

public string EmptyIfNull(object value)
    => value?.ToString() ?? string.Empty;