如何检查列表中是否已存在文件？

Question

This is the code:

for (int i = 0; i < files.Count; i++)
{
    if (pdf1.Lightnings.Count == 0)
    {
        pdf1.Lightnings.Add(files[i]);
    }
    if (files[i] != pdf1.Lightnings[i])
    {
        pdf1.Lightnings.Add(files[i]);
    }
}

Both files and Lightnings are List<string>

For example in files I have 33 indexes (files names) and I want to add them to the Lightnings List. But I want to check that if the file name from the List of files already exist in Lightnings so don't add it again.

The way it is now I'm getting error since when the variable i=1 so the line:

if (files[i] != pdf1.Lightnings[i])

Throws an error since in Lightnings i have only one index [0] and in i=1 already

Answer 1

You can use the .Contains method:

if (!pdf1.Lightnings.Contains(files[i]))
    pdf1.Lightnings.Add(files[i]);

This will check that files[i] does not already exist in the collection before adding.

Answer 2

You could try using distinct property and foreach loop:

var count = 0;
foreach(var file in files.Distinct())
{

    if (pdf1.Lightnings.Count == 0)
    {
        pdf1.Lightnings.Add(file);
    }
    if (files[i] != pdf1.Lightnings[count])
    {
        pdf1.Lightnings.Add(file);
    }
    count++;
}

Answer 3

pdf1.Lightnings.AddRange(files.Distinct());

要么

pdf1.Lightnings = pdf1.Lightnings.Union(files));

Answer 4

This can be done pretty easily using LINQ, it will also reduce the amount of code you need to write eg

var itemsToAdd = files.Where(x => !pdf.Lightnings.Contains(x));
pdf.Lightnings.AddRange(itemsToAdd);

Even one line if you still found it readable

pdf.Lightnings.AddRange(files.Where(x => !pdf.Lightings.Contains(x)));

Answer 5

As an alternative to the answers already given, if you want a unique list of items, you could use HashSets instead. Of course, whether the HashSet is better than a List depends on your usage, but it guarantees unique values.