In case if an argument is absent or invalid show description message …

DanTup.BrowserSelector/DanTup.BrowserSelector.csproj

@@ -57,6 +57,7 @@
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="RegistrySettings.cs" />
+    <Compile Include="UrlFixes.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="App.config" />

DanTup.BrowserSelector/Program.cs

@@ -21,7 +21,7 @@ static void Main(string[] args)
 
 			if (args == null || args.Length == 0)
 			{
-				ShowHelpInfo();
+				ShowHelpInfo("No arguments were passed to BrowserSelector");
 				return;
 			}
 
@@ -63,9 +63,12 @@ static void Main(string[] args)
 				}
 				else
 				{
-					if (arg.StartsWith("http://", StringComparison.OrdinalIgnoreCase) || arg.StartsWith("https://", StringComparison.OrdinalIgnoreCase) || arg.StartsWith("ftp://", StringComparison.OrdinalIgnoreCase))
+					if (arg.StartsWith("http:", StringComparison.OrdinalIgnoreCase) 
+                        || arg.StartsWith("https:", StringComparison.OrdinalIgnoreCase) 
+                        || arg.StartsWith("ftp:", StringComparison.OrdinalIgnoreCase))
 					{
-						LaunchBrowser(arg, waitForClose);
+                        var url = UrlFixes.AddMissedSlashesAfterProtocol(arg);
+						LaunchBrowser(url, waitForClose);
 					}
 					else if (arg.EndsWith(".url", StringComparison.InvariantCultureIgnoreCase) || arg.EndsWith(".website", StringComparison.InvariantCultureIgnoreCase))
 					{
@@ -77,16 +80,17 @@ static void Main(string[] args)
 					}
 					else
 					{
-						ShowHelpInfo();
+						ShowHelpInfo("The passed argument wasn't recognized as a valid url: " + arg);
 						return;
 					}
 				}
 			}
 		}
 
-		static void ShowHelpInfo()
+		static void ShowHelpInfo(string noArgumentReasonDescription = null)
 		{
-			MessageBox.Show(@"Usage:
+            var preMessage = string.IsNullOrEmpty(noArgumentReasonDescription) ? null : noArgumentReasonDescription + "\r\n\r\n";
+            MessageBox.Show(preMessage + @"Usage:
 
     BrowserSelector.exe --register
         Register as web browser

DanTup.BrowserSelector/UrlFixes.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DanTup.BrowserSelector
+{
+    public static class UrlFixes
+    {
+        public static string AddMissedSlashesAfterProtocol(string url)
+        {
+            var protocolEndPos = url.IndexOf(':');
+            if (protocolEndPos == -1)
+            {
+                throw new ArgumentException($"Not found a colon ':' after protocol in url: " + url);
+            }
+            if (protocolEndPos == url.Length)
+            {
+                //The passed url contains only a protocol. Just add slashes and return the result
+                return url + "//";
+                //throw new ArgumentException($"The passed url contains only a protocol: " + url);
+            }
+            var protocolLength = protocolEndPos + 1;
+            var urlAfterProtocol = url.Substring(protocolLength);
+            string slashesToAdd;
+            if (urlAfterProtocol[0] == '/')
+            {
+                if (urlAfterProtocol.Length == 1)
+                {
+                    //The passed url contains only a protocol with one slash. Just add one more slash and return the result
+                    return url + '/';
+                    //throw new ArgumentException($"The passed url contains only a protocol: " + url);
+                }
+                if (urlAfterProtocol[1] == '/')
+                {
+                    //all slashes are present. The passed url is valid so return it as is
+                    return url;
+                }
+                else
+                {
+                    slashesToAdd = "/";
+                }
+            }
+            else
+            {
+                slashesToAdd = "//";
+            }
+            var fixedUrl = url.Substring(0, protocolLength) + slashesToAdd + urlAfterProtocol;
+            return fixedUrl;
+        }
+    }
+}