add config element to load environment variables from a file

docs/xml-config-file.md

@@ -219,6 +219,14 @@ This optional element can be specified multiple times if necessary to specify en
 <env name="HOME" value="c:\abc" />
 ```
 
+### Environment File
+
+This optional element can be specified one time to specify a file to load environment variables from. Each variable definition must be on a separate line and in the format "key=value". Empty lines and lines starting with "#" are ignored. The syntax is:
+
+```xml
+<envFile>%BASE%/env.txt</env-file>
+```
+
 ### interactive
 
 If this optional element is specified, the service will be allowed to interact with the desktop, such as by showing a new window and dialog boxes.

src/WinSW.Core/Configuration/XmlServiceConfig.cs

@@ -78,6 +78,8 @@ namespace WinSW
             // Also inject system environment variables
             Environment.SetEnvironmentVariable(WinSWSystem.EnvVarNameServiceId, this.Name);
 
+            this.LoadEnvironmentVariablesFromFile();
+
             this.environmentVariables = this.LoadEnvironmentVariables();
         }
 
@@ -449,6 +451,11 @@ namespace WinSW
         /// </summary>
         public override Dictionary<string, string> EnvironmentVariables => this.environmentVariables;
 
+        /// <summary>
+        /// File from which environment variables are loaded.
+        /// </summary>
+        public string? EnvironmentVariablesFile => this.SingleElementOrNull("envFile");
+
         /// <summary>
         /// List of downloads to be performed by the wrapper before starting
         /// a service.
@@ -623,6 +630,37 @@ namespace WinSW
             return environment;
         }
 
+        private void LoadEnvironmentVariablesFromFile()
+        {
+            var envFile = this.SingleElementOrNull("envFile");
+
+            if (envFile is null)
+            {
+                return;
+            }
+
+            foreach (string line in File.ReadAllLines(envFile))
+            {
+                if (line.Length == 0 || line.StartsWith("#"))
+                {
+                    // ignore empty lines and comments
+                    continue;
+                }
+
+                int equalsSignIndex = line.IndexOf("=");
+
+                if (equalsSignIndex == -1)
+                {
+                    throw new WinSWException("The environment variables file contains one or more invalid entries. Each variable definition must be on a separate line and in the format \"key=value\".");
+                }
+
+                string key = line.Substring(0, equalsSignIndex);
+                string value = line.Substring(equalsSignIndex + 1);
+
+                Environment.SetEnvironmentVariable(key, value);
+            }
+        }
+
         private ProcessCommand GetProcessCommand(string name)
         {
             var node = this.root.SelectSingleNode(name);