How can I read an appSetting from web.config from .tt file (T4 Template)

Question

I am attempting to provide a URL in the web.config file that the build system will update before compiling the .tt files on my web project.

The relevant sections from my .tt file are below:

<#@ template debug="true" hostSpecific="true" language="C#" #>
<#@ output extension=".ts" #>
<#@ Assembly Name="System.Core.dll" #>
<#@ Assembly Name="System.Configuration.dll" #>
<#@ import namespace="System" #>
<#@ import namespace="System.Reflection" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="System.Text" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Configuration" #>
<#@ import namespace="System.Collections.Specialized" #>

import { Injectable } from "@angular/core";

<#= RegisterNameSpace("Server.Web.Dashboard.Models.APIRequest") #>
<#= RegisterNameSpace("Server.Web.Dashboard.Models.APIResponse") #>
<#= RegisterNameSpace("Server.Web.Dashboard.Models.APIGeneric") #>
<#= RegisterNameSpace("Server.Data.POCODataObjects") #>

@Injectable()
export class WebServiceCommunicatorService {

    rootUrl : string = '<#= ConfigurationManager.AppSettings["ServerServiceURL"] #>';

.....

When I try to generate the .ts file from the .tt file I get the following error message:

Severity    Code    Description Project File    Line    Suppression State
Error       An expression block evaluated as Null
System.ArgumentNullException: Value cannot be null.
Parameter name: objectToConvert
   at Microsoft.VisualStudio.TextTemplating.ToStringHelper.ToStringWithCulture(Object objectToConvert)
   at Microsoft.VisualStudio.TextTemplatingF553823B88CD6076D80EB08F6EA809752D0E5DC3E61C0FA53FB2F9AC22ACABF3B7BB9C8801A54E5DBC2A556C03FA42F2EA2AFCE58B708BEC94B0968193987868.GeneratedTextTransformation.TransformText() in webservicecommunicator.service.tt:line 35    Dashboard   webservicecommunicator.service.tt   35  

Answer 1

I've gone down the approach of pretending that the file is an XML file and not a config file. First i find the absolute path to the web.config, and then load it and read the node.

First the dependancies:

<#@ Assembly Name="System.Web.dll" #>
<#@ Assembly Name="System.Configuration.dll" #>
<#@ assembly name="System.Xml" #>

<#@ import namespace="System.Configuration" #>
<#@ import namespace="System.Web.Configuration" #>
<#@ import namespace="System.Collections.Specialized" #>
<#@ import namespace="System.Xml" #>
<#@ import namespace="System.IO" #>

So here is my attempt:

string GetWebServiceUrl() {
    XmlDocument doc = new XmlDocument();
    string absolutePath = this.Host.ResolvePath("../../../web.config");                
    doc.Load(absolutePath);
    XmlNode node = doc.DocumentElement.SelectSingleNode("/configuration/appSettings/add[@key='ServerServiceURL']");
    return node.Attributes["value"].Value.ToString();
}