C# how can I get all elements name from a xml file

Well, with XDocument and LINQ-to-XML:

foreach(var name in doc.Root.DescendantNodes().OfType<XElement>()
        .Select(x => x.Name).Distinct())
{
    Console.WriteLine(name);
}

There are lots of similar routes, though.

Using XPath

XmlDocument xdoc = new XmlDocument(); 
xdoc.Load(something);
XmlNodeList list = xdoc.SelectNodes("//BookStore");

gives you a list with all nodes in the document named BookStore

I agree with Adam, the ideal condition is to have a schema that defines the content of xml document. However, sometimes this is not possible. Here is a simple method for iterating all of the nodes of an xml document and using a dictionary to store the unique local names. I like to keep track of the depth of each local name, so I use a list of int to store the depth. Note that the XmlReader is "easy on the memory" since it does not load the entire document as the XmlDocument does. In some instances it makes little difference because the size of the xml data is small. In the following example, an 18.5MB file is read with an XmlReader. Using an XmlDocument to load this data would have been less effecient than using an XmlReader to read and sample its contents.

string documentPath = @"C:\Docs\cim_schema_2.18.1-Final-XMLAll\all_classes.xml";

Dictionary<string, List<int>> nodeTable = new Dictionary<string, List<int>>();
using (XmlReader reader = XmlReader.Create(documentPath))
{
    while (!reader.EOF)
    {
        if (reader.NodeType == XmlNodeType.Element)
        {
            if (!nodeTable.ContainsKey(reader.LocalName))
            {
                nodeTable.Add(reader.LocalName, new List<int>(new int[] { reader.Depth }));
            }
            else if (!nodeTable[reader.LocalName].Contains(reader.Depth))
            {
                nodeTable[reader.LocalName].Add(reader.Depth);
            }
        }
        reader.Read();
    }
}
Console.WriteLine("The node table has {0} items.",nodeTable.Count);
foreach (KeyValuePair<string, List<int>> kv in nodeTable)
{
    Console.WriteLine("{0} [{1}]",kv.Key, kv.Value.Count);
    for (int i = 0; i < kv.Value.Count; i++)
    {
        if (i < kv.Value.Count-1)
        {
            Console.Write("{0}, ", kv.Value[i]);
        }
        else
        {
            Console.WriteLine(kv.Value[i]);
        }
    }
}