SharePoint .NET CSOM code sample showing how to get ClientContext using "app only" authentication. (N.B. the app permissions must allow this)

GetAppOnlyClientContext.cs

// obtain web URL somehow - line below is fine for a list item RER, but you'll need another approach elsewhere.. 
string webUrl = properties.ItemEventProperties.WebUrl;

Uri webUri = new Uri(webUrl);

string realm = TokenHelper.GetRealmFromTargetUrl(webUri); 
string accessToken = TokenHelper.GetAppOnlyAccessToken(TokenHelper.SharePointPrincipal, webUri.Authority, realm).AccessToken; 
using (var clientContext = TokenHelper.GetClientContextWithAccessToken(webUrl, accessToken))  
{
    // do CSOM stuff here..
}