Add support for defaulting database, username, and password from envi…

…ronment variables (#526)

ClickHouse.Client/ADO/ClickHouseConnection.cs

@@ -37,9 +37,9 @@ public class ClickHouseConnection : DbConnection, IClickHouseConnection, IClonea
     private Version serverVersion;
     private string serverTimezone;
 
-    private string database = "default";
-    private string username;
-    private string password;
+    private string database = ClickHouseEnvironment.Database;
+    private string username = ClickHouseEnvironment.Username;
+    private string password = ClickHouseEnvironment.Password;
     private string session;
     private bool useServerTimezone;
     private bool useCustomDecimals;

ClickHouse.Client/ADO/ClickHouseConnectionStringBuilder.cs

@@ -17,19 +17,19 @@ public ClickHouseConnectionStringBuilder(string connectionString)
 
     public string Database
     {
-        get => GetStringOrDefault("Database", "default");
+        get => GetStringOrDefault("Database", ClickHouseEnvironment.Database);
         set => this["Database"] = value;
     }
 
     public string Username
     {
-        get => GetStringOrDefault("Username", "default");
+        get => GetStringOrDefault("Username", ClickHouseEnvironment.Username);
         set => this["Username"] = value;
     }
 
     public string Password
     {
-        get => GetStringOrDefault("Password", string.Empty);
+        get => GetStringOrDefault("Password", ClickHouseEnvironment.Password);
         set => this["Password"] = value;
     }
 

ClickHouse.Client/ADO/ClickHouseEnvironment.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ClickHouse.Client.ADO;
+
+internal static class ClickHouseEnvironment
+{
+    public static string Database => Environment.GetEnvironmentVariable("CLICKHOUSE_DB") ?? "default";
+
+    public static string Username => Environment.GetEnvironmentVariable("CLICKHOUSE_USER") ?? "default";
+
+    public static string Password => Environment.GetEnvironmentVariable("CLICKHOUSE_PASSWORD") ?? string.Empty;
+}