Just a quick one:
string firstline;
if (firstline == null) {
System.Console.WriteLine("String empty!");
}
In theory if there is no value in "firstline", the console should out put "String empty!"?
This doesn't even compile because of:
Use of unassigned local variable 'firstline'
When you say you don't get any output but the program compiles and runs just fine, you are not showing us your real code.
However, if firstline is not a local variable but a member variable of the surrounding class, it will be automatically initialized with null. In general, all member variables of a class are initialized with default(T) where T is the type of the member variable.
You can't compile in VS with error: Use of unassigned local variable 'firstline' !
Try assign null before!
EDIT
or
class Program
{
static string firstline; # with static you can compile and got your behaviour
static void Main(string[] args)
{
if (firstline == null)
{
System.Console.WriteLine("String empty!");
}
Console.ReadKey();
}
}
In default they are not null. If you want it null by default use this:
static string firstline;
static void Main(string[] args)
{
if (firstline == null)
{
System.Console.WriteLine("String empty!");
}
}
But I suggest using this one:
static void Main(string[] args)
{
string firstline = null;
// or this:
//string firstline = String.Empty;
if (String.IsNullOrEmpty(firstline))
{
System.Console.WriteLine("String empty!");
}
}
In both ways you can get riddle of
Use of unassigned local variable 'firstline'
Yes that will behave as you expect and should print out to the console. You might find that the console application closes too quickly for you to read the result though if you've not got any following code.
Also null and "" (or String.Empty) often mean the same thing, so a more command way to do this is:
if(String.IsNullOrEmpty(firstline))
{
Console.WriteLine("String is empty!");
}
Your terminology makes it sound like you are a little confused.
A null string is precisely that - the absence of the value. It is not an empty string, it is a string with no value.
An empty string is a zero length string, "" or string.Empty. This is not null as it does have a value, but the value is zero length.
Often you want to treat the null and empty values the same, in which case you might use the check
if (string.IsNullOrEmpty(firstline))
{
System.Console.WriteLine("String is null or empty!");
}
Your code is not correct, Use of unassigned local variable 'firstline'. You can assign it with any value to test. If you want to check if it is an empty string, a better way is:
string firstline = null; //change to "" to test
if (string.IsNullOrEmpty(firstline))
{
System.Console.WriteLine("String empty!");
}
here are my 2 cents:
if (firstline == (string)null) throw new ArgumentNullException("firstline"); //value is null
if (firstline.Length == 0) throw new ArgumentOutOfRangeException("firstline", "Value is empty"); // string.Empty
I found this by using Pex and Moles
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 have a regex that I am trying to evaluate the value of in the following method:
private void ValidateText()
{
if ((!IsOptional && Text == string.Empty))
{
SetIsValid(false);
}
else
{
if (this.ValidationRegex != null)
{
SetIsValid(this.ValidationRegex.IsMatch(this._text));
}
else
{
SetIsValid(true);
}
}
}
Where:
private Regex ValidationRegex { get; set; }
Now, when it gets to this.ValidationRegex != null I hover over the ValidationRegex value and {NULL} is displayed, so I expect my line to return false and go into the else statement.
Instead, this line returns true and I can't understand why. I have attached a screenshot of this below:
I can't seem to work it out (This is not a threading thing)
The pattern you have in the Regex object is NULL.
var ValidationRegex = new Regex("NULL");
This regex is looking for a string NULL in the input string (like This value is NULL.), and the ValidationRegex object is initialized and is not null. So, if evaluates to true.
Just a note: you can't check if a value is NULL using regex.
According to the documentation, the ToString() of Regex class (which is what Visual Studio is displaying) returns the regular expression pattern that was passed into the Regex constructor.
So I would assume your pattern is NULL.
Just like the title says.
I've tried doing str.Replace("","0"); but it gave me error because oldValue has zero length.
Is it possible to replace string.Empty into something?
Edit:
I am maintaining a program and I encountered that the program was calling a method that'll return a string then be converted to Int32.
int x = Convert.toInt32(Method1());
public string Method1()
{
string retString = string.Empty;
//Do Something
return retString
}
You can simply return "0" for null, zero length or whitespace string using this one-liner:
return String.IsNullOrWhiteSpace(str) ? "0" : str;
String.Replace takes two string arguments oldValue and newValue. You specified the newValue 0 however an empty string is not legal for the oldValue.
try below code :-
str.Replace(" ","0");
or you can just assign "0" to emptry string as below :-
if(str == string.Empty)
{
str = "0";
}
or making it simple :-
String.IsNullOrWhiteSpace(str) ? "0" : str;
You can't replace empty string within the string, but you can replace, say, spaces, e.g.
str = str.Replace(" ", "0"); // providing str is not null
Or you can substitute empty string with "0":
if (String.IsNullOrEmpty(str))
str = "0";
When parsing string into int you can do something like that:
int x = String.IsNullOrEmpty(str) ? 0 : Convert.ToInt32(str);
In method() you can do:
return String.IsNullOrEmpty(retString) ? "0" : retString;
If you want to check if the value is empty and then set the value to zero, otherwise use the default value you can use an inline if like so:
return string.IsNullOrWhiteSpace(retString ) ? "0" : retString;
If you know that str is empty then you may use star="a"
If you want to check then write statement in if condition.
After your edit:
To convert an empty string to 0, and parse a non-empty string as an integer, I wouldn't deal with a "0" at all, but combine the two in a single method. For example:
int Parse(string s, int d = 0) {
if (string.IsNullOrEmpty(s))
return d;
return int.Parse(s);
}
Try This...
public string Method1()
{
string retString = string.Empty;
//Do Something
return string.IsNullOrEmpty(retString)?"0":retString;
}
It is not possible to replace string.Empty by "0". It will throw ArgumentException.
An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll.
Additional information: String cannot be of zero length.
You can try following code:
if(retString == string.Empty)
{
retString = "0";
}
It sounds like your best option would be int.TryParse, if you encounter a string that can't be set to a valid value, it will set it to the default value for an integer (0) as well as returning a boolean so you can know this has happened.
int myInt;
if(!int.TryParse(myStringVariable, out myInt))
{
//Invalid integer you can put something here if you like but not needed
//myInt has been set to zero
}
//The same code without the if statement, still sets myInt
int.TryParse(myStringVariable, out myInt);
I am new to C# and I'm trying to write a contains statement. I want the process to read the test variable and if it contains the word error then print the variable if it does not contain an error then print no error. I think my process is close except when I run the code below I get an error when the CLI runs.
"object reference not set to an instance of an object"
Any help would be appreciated!
while (true)
{
test = queue.GetMessage();
if (test.AsString.Contains("error"))
{
Console.WriteLine(string.Format("Variable: {0}", test.AsString));
}
else
Console.WriteLine(string.Format("No Error: {0}", test.AsString));
}
var message = queue.GetMessage()?? string.Empty;
var formattedMessage =
String.Format(
(message.IndexOf("error", StringComparison.OrdinalIgnoreCase) >= 0)
? "No Error: {0}"
: "Variable: {0}", message);
Console.WriteLine(formattedMessage);
Useful references:
Unique ways to use the Null Coalescing operator
Simplify conditional string format
If queue.GetMessage() return a string, then you don't need AsString. If you want to convert it to a string, override ToString().
while (true) {
test = queue.GetMessage();
if (test.ToString().Contains("error")) {
...
} else {
...
}
}
You can always guarantee that ToString() will be present because it's defined in the object base class. Just be sure it returns something intelligible, because default implementations may not.
if i have a master property string for example:
public String SharedInfo
{
get { return (String)Session["SharedInfo"]; }
set { Session["SharedInfo"] = value; }
}
and a label in an content page, i check if the string is empty by doing:
if(Master.SharedInfo == null)
now my question is: why does if(Master.SharedInfo == "") not work, because the SharedInfo is a string right?
There is a handy method that "catches" both
if (String.IsNullOrEmpty(Master.SharedInfo)) {
...
}
null and "" are not equal. null means no string at all. "" is a string of length 0.
string s = null;
int i = s.Length; // <-- Throws null reference exception
But
string s = "";
int i = s.Length; // OK, i => 0
"" and String.Empty are equivalent. Some people state that you should always use String.Empty instead of "", but it makes really no difference.
UPDATE
Equal string constants are interned by the compiler, i.e. the compiler stores equal constants only once. You can make a simple test (in response to #BobTodd's comment),
string s = "";
Console.WriteLine(Object.ReferenceEquals(s, "")); // --> true
Console.WriteLine(Object.ReferenceEquals(s, String.Empty)); // --> true
For the sake of completeness (according to #JoelEtherton's comment). Starting from .NET Framework 4.0 you can test
if (String.IsNullOrWhitespace(Master.SharedInfo)) {
...
}
This will catch strings like " " or "\t" as well.
In c#, an empty string "" is not null. It's an actual string, with length equals to zero.
You can use string.IsNullOrEmpty(string stringToTest) to check both null and empty strings.
String.Empty ("") and null are quite different
It depends wholly on what has been written to Session["SharedInfo"].
If nothing has, then it will be null, otherwise its the value written, which could be an empty string or null.
To be sure use String.IsNullOrEmpty(string #string)