How to check to see if my variable contains a string

Question

In my c# program, I'm reading in lines from a text file and want to check to see if the line that I read in contains the string "EHRS" and if it does, I want to put that line into an array and if not, I want to read in the next line. When I run this, I get an error that tells me that an unhandled exception of type 'System.NullReferenceException' occurred at the line where I use the contains function.

Here's what I have:

string[] concArray = new string[numLinesConcYr];

using (var sr = new StreamReader(mydocpath + @"\concYrLines.txt"))
{
    for (int i = 0; i < numLinesConcYr; i++)
    {
        concYrLine = sr.ReadLine();
        if (concYrLine.Contains("EHRS") == true)
        {
            concArray[i] = concYrLine;
        }
        else
        {
            concYrLine = sr.ReadLine();
        }
    }
}

Tigran · Accepted Answer · 2018-04-05T18:23:57.260

2

From documentation on StreamReader.ReadLine

Return Value Type: System.String The next line from the input stream, or null if the end of the input stream is reached.

So null is expected, you have to implement defense against it.

concYrLine = sr.ReadLine();
if (!string.IsNullOrempty(concYrLine) && concYrLine.Contains("EHRS"))
{
}

edited Apr 05 '18 at 18:23

answered Apr 05 '18 at 18:17

Tigran

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8
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The thing is though, it's not null. Whenever I just read in all of the lines into the array and print them out, it prints out all of the lines perfectly – rcarcia02 Apr 05 '18 at 18:19
@asdf: clearly your `numLinesConcYr` is bigger than number of lines in file. – Tigran Apr 05 '18 at 18:19
I use a loop to go through my file and count the lines though so they should be the same – rcarcia02 Apr 05 '18 at 18:21
You got a NullReferenceException, so clearly `concYrLine` is null. – Dan Wilson Apr 05 '18 at 18:21
@asdf: to be sure, for sake of test, try to call `File.ReadAllLines()` and check the number of elements in resulting array – Tigran Apr 05 '18 at 18:22
@Rahul: yep, typo. thanks – Tigran Apr 05 '18 at 18:24
1

you can as well use a null propagation operator to avoid null facing `concYrLine?.Contains("EHRS")` – Rahul Apr 05 '18 at 18:25
whenever I use File.ReadAllLines(), the number of elements is the same and whenever I print out all of the lines none of them are empty – rcarcia02 Apr 05 '18 at 18:26
@asdf: is the count of line read the same you have it in `numLinesConcYr` variable ? – Tigran Apr 05 '18 at 18:27
the count of lines read is the same as I have in the numLines variable – rcarcia02 Apr 05 '18 at 18:27
@asdf: another option would be to debug and see on which iteration of loop result is `null` – Tigran Apr 05 '18 at 18:28

score 0 · Answer 2 · answered Apr 05 '18 at 18:17

0

Each time you read a line, your line my have no content. performing a .Contains("") check on the line you have read is dependent on the variable returned being an instance of a string. Perform a null check on the concYrLine proir to the .Contains() method. Like so:

for (int i = 0; i < numLinesConcYr; i++)
{
    concYrLine = sr.ReadLine();
    if (!String.IsNullOrEmpty(concYrLine) && concYrLine.Contains("EHRS") == true)
    {
        concArray[i] = concYrLine;
    }
    else
    {
        concYrLine = sr.ReadLine();
    }
}

answered Apr 05 '18 at 18:17

Andy Clark

3,363
7
27
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The thing is though, it's not null. Whenever I just read in all of the lines into the array and print them out, it prints out all of the lines perfectly – rcarcia02 Apr 05 '18 at 18:19
Also, `concYrLine.Contains("EHRS") == true` is verbose, there is no need for the `== true` as the `Contains` method returns a boolean already – Andy Clark Apr 05 '18 at 18:19
My Assumption is it must be null, as this article which explains why the exception is thrown https://msdn.microsoft.com/en-us/library/dy85x1sa(v=vs.110).aspx When you say you "print them out", where are you printing them? – Andy Clark Apr 05 '18 at 19:06

How to check to see if my variable contains a string

2 Answers2