diff --git a/wfassoc/src/highlevel.rs b/wfassoc/src/highlevel.rs
index 6b3834c..b2a86f0 100644
--- a/wfassoc/src/highlevel.rs
+++ b/wfassoc/src/highlevel.rs
@@ -1,4 +1,7 @@
-use crate::{lowlevel, utilities, win32::{self, concept}};
+use crate::{
+    lowlevel, utilities,
+    win32::{self, concept},
+};
 use regex::Regex;
 use std::collections::HashMap;
 use std::ffi::OsStr;
@@ -462,9 +465,12 @@ impl Program {
         debug_println!("Adding ProgId subkey...");
         for program_progid_key in &mut self.ext_keys {
             let progid_key = &mut program_progid_key.progid_key;
-            
+            debug_println!(
+                "Adding ProgId \"{0}\" subkey...",
+                progid_key.inner().to_string()
+            );
+
             // Create ProgId subkey
-            debug_println!("Adding ProgId \"{0}\" subkey...", progid_key.inner().to_string());
             progid_key.ensure(scope)?;
             // Write ProgId values
             let name = Some(&program_progid_key.name.inner);
@@ -472,6 +478,10 @@ impl Program {
             progid_key.set_shell_verb(scope, &program_progid_key.behavior.inner)?;
             progid_key.set_friendly_type_name(scope, name)?;
             progid_key.set_default_icon(scope, Some(&program_progid_key.icon.inner))?;
+
+            // Add this progid to file extension "open with" list.
+            let ext_key = &mut program_progid_key.ext_key;
+            ext_key.add_into_open_with_progids(scope, progid_key.inner())?;
         }
 
         // Everything is okey.
@@ -488,24 +498,33 @@ impl Program {
         // Delete App Paths subkey
         debug_println!("Deleting App Paths subkey...");
         self.app_paths_key.delete(scope)?;
-        
+
         // Delete Applications subkey
         debug_println!("Deleting Applications subkey...");
         self.applications_key.delete(scope)?;
 
-        // According to Microsoft document, when uninstalling application,
-        // it is enough that only delete ProgId keys.
-        // Programmer doesn't need to edit something in Ext keys.
-        // So we delete ProgId subkeys one by one in there.
+        // Delete ProgId subkeys one by one.
         debug_println!("Adding ProgId subkey...");
         for program_progid_key in &mut self.ext_keys {
             let progid_key = &mut program_progid_key.progid_key;
-            
+            debug_println!(
+                "Deleting ProgId \"{0}\" subkey...",
+                progid_key.inner().to_string()
+            );
+
+            // YYC MARK:
+            // According to Microsoft document, when uninstalling application,
+            // there is no need to reset the default open way of file extension.
+            // So we simply remove it from "open with" list.
+
+            // Remove this ProgId from file extension "open with" list.
+            let ext_key = &mut program_progid_key.ext_key;
+            ext_key.remove_from_open_with_progids(scope, progid_key.inner())?;
+
             // Delete ProgId subkey
-            debug_println!("Deleting ProgId \"{0}\" subkey...", progid_key.inner().to_string());
             progid_key.delete(scope)?;
         }
-        
+
         // Everything is okey.
         // Notify changes and return
         win32::utilities::notify_assoc_changed();
@@ -517,8 +536,35 @@ impl Program {
     /// Please note that this is a rough check and do not validate any data.
     ///
     /// The return value only ensures the pre-requirement of `register` and `unregister`.
-    pub fn is_registered(&self, view: View) -> Result<bool, ProgramError> {
-        todo!()
+    pub fn is_registered(&self, scope: Scope) -> Result<bool, ProgramError> {
+        // Check App Paths subkey.
+        debug_println!("Checking App Paths subkey...");
+        if !self.app_paths_key.is_exist(scope)? {
+            return Ok(false);
+        }
+
+        // Check Application subkey.
+        debug_println!("Checking Applications subkey...");
+        if !self.applications_key.is_exist(scope.into())? {
+            return Ok(false);
+        }
+
+        // Check ProgId subkey.
+        debug_println!("Checking ProgId subkey...");
+        for program_progid_key in &self.ext_keys {
+            let progid_key = &program_progid_key.progid_key;
+            debug_println!(
+                "Checking ProgId \"{0}\" subkey...",
+                progid_key.inner().to_string()
+            );
+
+            if !progid_key.is_exist(scope.into())? {
+                return Ok(false);
+            }
+        }
+
+        // Every subkeys are roughly existing.
+        Ok(true)
     }
 
     pub fn link_ext(&self, scope: Scope, index: usize) -> Result<(), ProgramError> {