Sample code for calling SharePoint CSOM from an Azure Function

csomHelper.csx

using Microsoft.IdentityModel.Clients.ActiveDirectory;
using Microsoft.SharePoint.Client;
using System.Security.Cryptography.X509Certificates;

public static class csomHelper {

    private static string ClientId = "<fill in app ID>";
    private static string Cert = "myCert.pfx";                      // Fill in name of your cert file and upload it
    private static string CertPassword = "<fill in cert password>"; // TODO: Explore more secure place for this
    private static string Authority = "https://login.windows.net/<tenant>.onmicrosoft.com/";
    private static string Resource = "https://<tenant>.sharepoint.com/";

    public async static Task<ClientContext> GetClientContext(string siteUrl)
    {
        var authenticationContext = new AuthenticationContext(Authority, false);

        // TODO: Substitute your Azure function name for GetDocUrl2 below:
        var certPath = Path.Combine(Environment.GetEnvironmentVariable("HOME"), "site\\wwwroot\\GetDocUrl2\\", Cert);
        var cert = new X509Certificate2(System.IO.File.ReadAllBytes(certPath),
            CertPassword,
            X509KeyStorageFlags.Exportable |
            X509KeyStorageFlags.MachineKeySet |
            X509KeyStorageFlags.PersistKeySet);

        var authenticationResult = await authenticationContext.AcquireTokenAsync(Resource, new ClientAssertionCertificate(ClientId, cert));
        var token = authenticationResult.AccessToken;

        var ctx = new ClientContext(siteUrl);
        ctx.ExecutingWebRequest += (s, e) =>
        {
            e.WebRequestExecutor.RequestHeaders["Authorization"] = "Bearer " + authenticationResult.AccessToken;
        };

        return ctx;
    }
}

function.json

{
  "bindings": [
    {
      "authLevel": "function",
      "name": "req",
      "type": "httpTrigger",
      "direction": "in",
      "methods": [
        "get"
      ]
    },
    {
      "name": "$return",
      "type": "http",
      "direction": "out"
    }
  ],
  "disabled": false
}

project.json

{
  "frameworks": {
    "net46":{
      "dependencies": {
        "Microsoft.IdentityModel.Clients.ActiveDirectory" : "3.13.7",
        "Microsoft.SharePointOnline.CSOM": "16.1.5813.1200"
      }
    }
   }
}

run.csx

#load "csomHelper.csx"

using System.Net;
using Microsoft.SharePoint.Client;
using Microsoft.SharePoint.Client.Utilities;

public static async Task<HttpResponseMessage> Run(HttpRequestMessage req, TraceWriter log)
{
    ClientResult<string> result;
    try {
        log.Info("Executing function");

        // Parse query parameters
        string path = req.GetQueryNameValuePairs()
            .FirstOrDefault(q => string.Compare(q.Key, "path", true) == 0)
            .Value;
        string siteUrl = req.GetQueryNameValuePairs()
            .FirstOrDefault(q => string.Compare(q.Key, "siteUrl", true) == 0)
            .Value;

        // Get request body
        dynamic data = await req.Content.ReadAsAsync<object>();

        // Build item URL
        string itemUrl = (siteUrl ?? data?.siteUrl).TrimEnd('/') + "/" + (path ?? data?.path).TrimStart('/');
        string serverRelativeUrl = itemUrl.Substring(itemUrl.IndexOf('/',9));

        // Get Office Online (WOPI) URL
        using (var ctx = await csomHelper.GetClientContext(siteUrl))
        {
            File f = ctx.Web.GetFileByServerRelativeUrl (serverRelativeUrl);
            result = f.ListItemAllFields.GetWOPIFrameUrl(SPWOPIFrameAction.View);

            ctx.Load(f.ListItemAllFields);
            ctx.ExecuteQuery();

        }

        // Return item URL or error
        return itemUrl == null
            ? req.CreateResponse(HttpStatusCode.BadRequest, "Please pass a path on the query string or in the request body" + req.RequestUri) 
            : req.CreateResponse(HttpStatusCode.OK, result.Value);
    }
    catch (Exception ex)
    {
        string message = ex.Message + "\n" + ex.StackTrace;
        return req.CreateResponse(HttpStatusCode.BadRequest, "ERROR: " + message);
    }
}