Check if string is null without erroring in c#

Question

In the line 3 Im getting null exception. 'Discription' is a string were value sometimes is null. what check I can do before it goes to - (!Discription.ToLower().StartsWith("da") and throws error? Or how do I make null value acceptable?

private void FindBook(a, b)
{
    if (discription == null && (!Discription.ToLower().StartsWith("da")))  //getting 'Can't be NULL' exception here.How do approach this issue? How do I fix this?
    {
        IsInLibrary = false;
    }
    else
    {
        IsInLibrary = true;
    }
}

public string Discription
{
    get
    {
        return _discription;
    }

    set
    {
        _discription = value;

        if (value != null && (value.ToLower().StartsWith("da"))) //it is working here 
        {
            IsInLibrary = true;
        }

        OnPropertyChanged("Discription");
    }
}

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– Some programmer dude, Commented Apr 22, 2021 at 8:23
Your 1st and 2nd checks for null are not the same, the 1st is wrong, the 2nd is correct as you already found (where it says //it is working here). Just compare them character by character... — Peter B
– Peter B, Commented Apr 22, 2021 at 8:31
You have discription, Discription and _discription do you need all 3? — tmaj
– tmaj, Commented Apr 22, 2021 at 8:31
this is if it is not null but I need it for null. after I need to do check for (!Discription.ToLower().StartsWith("da"))) which is erroring because it is null. If I take a way '!' it errors — Laura
– Laura, Commented Apr 22, 2021 at 8:33

tmaj · Accepted Answer · 2021-04-22 08:46:24Z

1

In Summary, your reversed the parts but you didn't change && to ||.

I think your question is about reversing logic from

if (x != null && x.ToLower().StartsWith("da"))
{
   y = true;
}

to

if (???)
{
   y = false;
}

Option A. Reverse all of it

You can achieve it with ! around the whole content of if:

if (!(x != null && x.Something()))
{
  y = false;
}

Option B. De Morgan's laws

De Morgan's laws can be expressed as

not (A or B) = not A and not B
not (A and B) = not A or not B.

We want to use the 2nd law:

not (A and B) = not A or not B

In our case:

A => (x != null)
B => (x.ToLower().StartsWith("da"))

Then:

not ((x != null) and (x.ToLower().StartsWith("da")))

is equal to

not (x != null) or not (x.ToLower().StartsWith("da")

This can be simplified to:

x == null or !x.ToLower().StartsWith("da")

and in C#:

x == null || !x.ToLower().StartsWith("da")

answered Apr 22, 2021 at 8:46

tmaj

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2 Comments

Fildor Over a year ago

OP: Mind that x == null || !x.ToLower().StartsWith("da") works, because the second clause won't be evaluated if the first one resolves to true.

tmaj Over a year ago

@Fildor True and the same applies to x != null && x.ToLower()...

sommmen · Accepted Answer · 2021-04-22 08:27:29Z

0

You're getting a nullref because you are checking whether the variable is null, not that it isnt.

Change:

if (discription == null && (!Discription.ToLower().StartsWith("da")))

To:

if (discription != null && (!Discription.ToLower().StartsWith("da")))

Aside from thise, people prefer to use string.IsNullOrEmpty(str) or string.IsNullOrWhiteSpace(str) to also check for empty or whitespace only strings, because often those are also invalid.

answered Apr 22, 2021 at 8:27

sommmen

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Comments

AL.Sharie · Accepted Answer · 2021-04-22 08:29:30Z

0

you can use this

if (!string.IsNullOrWhiteSpace(value)  && (value.ToLower().StartsWith("da")))

answered Apr 22, 2021 at 8:29

AL.Sharie

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1 Comment

Klamsi Over a year ago

Or a more hackish short version: if ((value ?? "").ToLower().StartsWith("da"))

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Check if string is null without erroring in c#

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Option A. Reverse all of it

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