Error: Collection modified; maybe enumeration operation is not executed

Question

I have a problem using the foreach to traverse a array of strings.

This component is a search button to select files, it selects and writes the address of the file into a array of strings, i created a repeat loop to check if the same file was selected twice, but the second run of loop results in an exception.

How the exception is generated:

1. I select two files, they are stored in array hassle-free.
2. I again select the same two files to test the method.
3. The first check is done successfully, the program recognizes that the file is already in the list and does not add it.
4. When checking the second file the exception is generated.

    List<string> ListPath = new List<string>(); // Array
    string File; // Arquivo

    private void Search_Click(object sender, EventArgs e)
    {
        OpenFileDialog AbrirArquivo = new OpenFileDialog(); //Instância
        AbrirArquivo.Multiselect = true; // habilita a multseleção de arquivos
        if (AbrirArquivo.ShowDialog() == DialogResult.OK) // ao pressionar ok
        {
            foreach (var File in AbrirArquivo.FileNames) // para cada arquivo no array
            {
                if(ListPath.Count == 0) // se a lista for vazia
                {
                    ListPath.Add(File); //adiciona o primeiro elemento
                }
                else // se não for vazia, agora ele verifica se já existe
                {
                    foreach(var item in ListPath) //Onde ocorre a Exception 
                    {
                        if(item == File) //se o item da lista for igual ao caminho do arquivo
                        {
                            continue; // pula para o próximo e não adiciona na lista
                        }
                        else
                        {
                            ListPath.Add(File); // se não, adiciona.
                        }
                    }
                }

            }
        }
        PathText.Text = (ListPath.Count + " Arquivos selecionados"); 
// mostra em um textBox quantos arquivos foram selecionados.
    }

Answer 1

Why not use LINQ which is much easier and avoids these errors?

listPath.AddRange(AbrirArquivo.FileNames.Distinct().ToList());

I put in the Github for future reference.