Reading files from a folder and converting to Json in C#

Question

I have the code below,after retrieving the file i want to convert it to json.

Any help is appreciated.

public string DownloadDocument(string documentname, string documentlocation)
{
string obj = string.Empty;
DirectoryInfo d = new DirectoryInfo(documentlocation);
FileInfo[] Files = d.GetFiles();
foreach (FileInfo file in Files)
{
if (file.Name.Contains(documentname))
{
//want to convert file to json as below but not working
//obj = Newtonsoft.Json.JsonConvert.SerializeObject(file);
}
}
return obj;
}