Try to use FileInformation class to get the file stream:
var web = clientContext.Web;
clientContext.Load(web, website => website.ServerRelativeUrl);
clientContext.ExecuteQuery();
var regex = new Regex(SiteUrl, RegexOptions.IgnoreCase);
var siteRelavtiveURL = regex.Replace(path, string.Empty);
var serverRelativeURL = web.ServerRelativeUrl + siteRelavtiveURL;
var file = web.GetFileByServerRelativeUrl(serverRelativeURL);
clientContext.Load(file);
clientContext.ExecuteQuery();
FileInformation fileInfo = Microsoft.SharePoint.Client.File.OpenBinaryDirect(clientContext, serverRelativeURL);
using (var memoryStream = new MemoryStream())
{
fileInfo.Stream.CopyTo(memoryStream);
return memoryStream.ToArray();
}
Method “OpenBinaryStream” does not exist reading a file from SharePoint 2010
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