[dotnet] allow user to start service before creating driver

dotnet/src/webdriver/Chrome/ChromeDriverService.cs

@@ -41,6 +41,12 @@ private ChromeDriverService(string executablePath, string executableFileName, in
         {
         }
 
+        /// <inheritdoc />
+        protected override DriverOptions GetDefaultDriverOptions()
+        {
+            return new ChromeOptions();
+        }
+
         /// <summary>
         /// Creates a default instance of the ChromeDriverService.
         /// </summary>

dotnet/src/webdriver/DriverService.cs

@@ -262,6 +262,11 @@ public void Start()
                 return;
             }
 
+            if (this.driverServicePath == null)
+            {
+                DriverFinder.FullPath(this.GetDefaultDriverOptions());
+            }
+
             this.driverServiceProcess = new Process();
             this.driverServiceProcess.StartInfo.FileName = Path.Combine(this.driverServicePath, this.driverServiceExecutableName);
             this.driverServiceProcess.StartInfo.Arguments = this.CommandLineArguments;
@@ -283,6 +288,12 @@ public void Start()
             }
         }
 
+        /// <summary>
+        /// The browser options instance that corresponds to the driver service
+        /// </summary>
+        /// <returns></returns>
+        protected abstract DriverOptions GetDefaultDriverOptions();
+
         /// <summary>
         /// Releases all resources associated with this <see cref="DriverService"/>.
         /// </summary>

dotnet/src/webdriver/Edge/EdgeDriverService.cs

@@ -41,6 +41,12 @@ private EdgeDriverService(string executablePath, string executableFileName, int
         {
         }
 
+        /// <inheritdoc />
+        protected override DriverOptions GetDefaultDriverOptions()
+        {
+            return new EdgeOptions();
+        }
+
         /// <summary>
         /// Gets or sets a value indicating whether the service should use verbose logging.
         /// </summary>

dotnet/src/webdriver/Firefox/FirefoxDriverService.cs

@@ -50,6 +50,12 @@ private FirefoxDriverService(string executablePath, string executableFileName, i
         {
         }
 
+        /// <inheritdoc />
+        protected override DriverOptions GetDefaultDriverOptions()
+        {
+            return new FirefoxOptions();
+        }
+
         /// <summary>
         /// Gets or sets the location of the Firefox binary executable.
         /// </summary>

dotnet/src/webdriver/IE/InternetExplorerDriverService.cs

@@ -48,6 +48,12 @@ private InternetExplorerDriverService(string executablePath, string executableFi
         {
         }
 
+        /// <inheritdoc />
+        protected override DriverOptions GetDefaultDriverOptions()
+        {
+            return new InternetExplorerOptions();
+        }
+
         /// <summary>
         /// Gets or sets the value of the host adapter on which the IEDriverServer should listen for connections.
         /// </summary>

dotnet/src/webdriver/Safari/SafariDriverService.cs

@@ -46,6 +46,12 @@ private SafariDriverService(string executablePath, string executableFileName, in
         {
         }
 
+        /// <inheritdoc />
+        protected override DriverOptions GetDefaultDriverOptions()
+        {
+            return new SafariOptions();
+        }
+
         /// <summary>
         /// Gets or sets a value indicating whether to use the default open-source project
         /// dialect of the protocol instead of the default dialect compliant with the