I have many scenarios in my application where I am declaring strings as string.empty and later dynamically adding values to it. In C#, Is
string status
and
string status = String.Empty;
same?
Those lines of code are not equivalent.
If you've declared string status outside of a method, it initializes to its default value of null.
If you've declared string status inside a method, it isn't initialized, and you can't use it until you explicitly give it a value.
Whether or not you need string status = String.Empty; depends on your situation, but it seems like a decent way of avoiding a NullReferenceException if you find your code sometimes throws.
No. It's not the same. String datataype allows null. And remember that it is encouraged that you always initialize all your variables/attributes/properties.
string status = String.Empty;
No, the default value of string variable is Null
string status;
when inside a method: it would stay uninitialized
when outside a method: it would create a string object with a Null value, because string is a reference type.
string status = String.Empty;
will create a string object with a value of the Empty constant which is a string of zero length
Related
New to C#. I am trying this code, which I copied from w3school User Input
using System;
namespace Sample
{
class Test
{
public static void Main(string[] args)
{
string testString;
Console.Write("Enter a string - ");
testString = Console.ReadLine();
Console.WriteLine("You entered '{0}'", testString);
}
}
}
But after I execute the Program, it shows an error at Console.ReadLine();
Converting null literal or possible null value to non-nullable type.
you need to handle nullable try to replace
string testString;
with
string? testString = null;
or
string testString="";
In the declaration of testString, take note of the ? symbol that has been added after the data type. This shows that the variable can have a null value, which means it is nullable.
In recent versions of .NET, nullable reference types are supported by default and Console.ReadLine returns a nullable string. That means that you need to either declare your variable as nullable:
string? testString;
or specify that the return value will definitely not be null:
testString = Console.ReadLine()!;
If you use the former, you'll have allow for that when you use the variable later. The latter is OK because, in a console window, it will never be null.
You ought to do some reading on nullable reference types.
You need to make sure that you have a valid non-null string.
You could do this for example to make sure that it's valid before printing it to the console using the formatter:
testString = Console.ReadLine() ?? string.Empty;
This will assign an empty string to the testString variable in case that ReadLine() returns null.
I am new to c# so i need some guidance with this.
string NumInput = Console.ReadLine();
in my code, this statement gives the warning of
converting null literal or possible null value to non-nullable type.
Is there any alternative to this code line or anything which can make this warning disapear
Firstly, you are seeing this message because you have the C# 8 Nullable reference type feature enabled in your project.
Console.ReadLine() returns a value that can be either a string or null but you are using it to set a variable of type string. Instead either use the var keyword which will implicitly set your variable to the same type as the return value or manually set your variable as type nullable string string?. the ? denotes a nullable reference type.
You may then want to check that your variable does infact hold a string before you use it:
string? NumInput = Console.ReadLine();
if (NumInput == null)
{
// ...
}
In C# 6.0, string interpolations are added.
string myString = $"Value is {someValue}";
How are null values handled in the above example? (if someValue is null)
EDIT:
Just to clarify, I have tested and am aware that it didn't fail, the question was opened to identify whether there are any cases to be aware of, where I'd have to check for nulls before using string interpolation.
That's just the same as string.Format("Value is {0}", someValue) which will check for a null reference and replace it with an empty string. It will however throw an exception if you actually pass null like this string.Format("Value is {0}", null). However in the case of $"Value is {null}" that null is set to an argument first and will not throw.
From TryRoslyn, it's decompiled as;
string arg = null;
string.Format("Value is {0}", arg);
and String.Format will use empty string for null values. In The Format method in brief section;
If the value of the argument is null, the format item is replaced with
String.Empty.
It seems that the behavior depends on which underlying formatting methods are called, and the implementation of these can change over time. If you get a null formated into the string such as "(null)", it is not sure this will stay the same over several years. In some newer version of .NET it can start throwing an exception.
So I think the most safe approach is to make some condition to avoid using the null. Write a simple ternary operation like:
int? someValue = 5;
var valueStr = (someValue is not null) ? someValue.ToString() : string.Empty;
var myString = $"Value is {valueStr}";
It is an extra line of code, but at least the behavior is controlled.
I've made a method to convert a datetime object to a date-only format. In order to access it, I have to assign the value of my parameter to the value that will be in a datatable. The obvious method would thus be to add a ? after the DateTime object to make it nullable, however this removes it's methods (or at least, the one I need to use), making my method worthless.
string dateWithOutTime(DateTime? datetime)
{
string reply = datetime.Date.ToString();
return reply;
}
usage:
string str = dateWithOutTime(*DataSet*.*DataTable[n]*.Rows[n][n] as DateTime?);
Is there a way to accomplish this without adding any extra objects?
Note: a star (*) denotes a variable type/object
DateTime? doesn't have the same methods as DateTime, they're different types. You have to retrieve the actual datetime value using DateTime?'s Value property:
string dateWithOutTime(DateTime? datetime)
{
if(datetime.HasValue)
return datetime.Value.Date.ToString();
else
return //...
}
Read the documentation for Nullable<T> here.
Unless I misunderstand your problem, I'd say you need to check if your DateTime? parameter is null or not. If it is, return an empty string (or a string you want to display for missing dates). If it isn't, you can use the Value property:
string dateWithOutTime(DateTime? datetime)
{
return datetime.HasValue ? datetime.Value.Date.ToString() : String.Empty;
}
Update
If you only want the date-part in your string and you want it to be culture-sensitive, you can use ToShortDateString() instead of ToString(). You can even leave out the Date property:
string dateWithOutTime(DateTime? datetime)
{
return datetime.HasValue
? datetime.Value.ToShortDateString()
: String.Empty;
}
I was reading this blog post by Eric Lippert http://ericlippert.com/2013/06/17/string-concatenation-behind-the-scenes-part-one/#more-1228 and became aware that the empty string is not an identity of concatenation in C#. I haven't run into a situation that made me aware this was the case and always just assumed it was an identity. I assume there is some good reason why
string NullString = null;
NullString = NullString + String.Empty; // results in and empty string, not null
results in an empty string rather than null, what is that reason? Why is there no identity of string concatenation? Was it made that way for convenience or practicality?
The documentation for String.Concat explains this behavior:
An Empty string is used in place of any null argument.
Basically, the String.Concat method was designed to exhibit this behavior.
Was it made that way for convenience or practicality?
While only the framework design team could directly answer this, this behavior does has some practical benefits. This behavior allows you to concatenate strings with null and not create null results, which reduces the number of explicit null checks required in most code. Without this behavior, someString + "abc" would require null checking, wheras with it, a non-null value is guaranteed.
I must admit that i don't understand "the identity of string concatenation". However, the reason why null + string.Empty is not null but string.Empty is:
because it was implemented in this way.
Have a look:
public static string Concat(string str0, string str1)
{
if (string.IsNullOrEmpty(str0))
{
if (string.IsNullOrEmpty(str1))
{
return string.Empty;
}
return str1;
}
else
{
if (string.IsNullOrEmpty(str1))
{
return str0;
}
int length = str0.Length;
string text = string.FastAllocateString(length + str1.Length);
string.FillStringChecked(text, 0, str0);
string.FillStringChecked(text, length, str1);
return text;
}
}
This is also documented:
The method concatenates str0 and str1; it does not add any delimiters.
An Empty string is used in place of any null argument.
If you ask for the why. I assume because it's safer this way. If you want to concat two strings and one of both is null, why should null be favored instead of string.Empty?
Because it uses a contract, the purpose of which is described at Code Contracts.
From String.Concat:
Contract.Ensures(Contract.Result<string>() != null);
Note that NullString + NullString also returns an empty string.