diff --git a/wfassoc/src/assoc.rs b/wfassoc/src/assoc.rs
index 24d27fb..a461272 100644
--- a/wfassoc/src/assoc.rs
+++ b/wfassoc/src/assoc.rs
@@ -4,7 +4,6 @@
 use crate::extra::windows::{Ext, ProgId};
 use crate::extra::winreg as winreg_extra;
 use crate::utilities;
-use std::convert::Infallible;
 use std::fmt::Display;
 use std::str::FromStr;
 use thiserror::Error as TeError;
@@ -25,7 +24,9 @@ pub enum Error {
     #[error("{0}")]
     ParseProgId(#[from] crate::extra::windows::ParseProgIdError),
     #[error("{0}")]
-    BlankPath(#[from] crate::extra::winreg::BlankStringError),
+    ParseProgIdKind(#[from] ParseProgIdKindError),
+    #[error("{0}")]
+    BlankPath(#[from] crate::extra::winreg::BlankPathError),
 }
 
 /// The result type used in this crate.
@@ -113,6 +114,15 @@ impl From<Scope> for View {
 
 // region: ProgId Kind
 
+/// The error occurs when parsing ProgId kind.
+#[derive(Debug, TeError)]
+pub enum ParseProgIdKindError {
+    #[error("{0}")]
+    FromStd(#[from] crate::extra::windows::ParseProgIdError),
+    #[error("given ProgId is blank")]
+    BlankProgId,
+}
+
 /// The variant of ProgId for the compatibility
 /// with those software which do not follow Microsoft suggestions.
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -133,13 +143,17 @@ impl Display for ProgIdKind {
 }
 
 impl FromStr for ProgIdKind {
-    type Err = Infallible;
+    type Err = ParseProgIdKindError;
 
     fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
-        Ok(match s.parse::<ProgId>() {
-            Ok(v) => Self::Std(v),
-            Err(_) => Self::Other(s.to_string()),
-        })
+        if s.is_empty() {
+            Err(ParseProgIdKindError::BlankProgId)
+        } else {
+            Ok(match s.parse::<ProgId>() {
+                Ok(v) => Self::Std(v),
+                Err(_) => Self::Other(s.to_string()),
+            })
+        }
     }
 }
 
@@ -231,6 +245,20 @@ pub struct ExtKey {
     ext: Ext,
 }
 
+impl ExtKey {
+    /// Create new file extension registry key representer.
+    pub fn new(s: &str) -> Result<Self> {
+        Ok(Self {
+            ext: Ext::from_str(s)?,
+        })
+    }
+
+    /// Fetch the reference to inner extension representer.
+    pub fn as_inner(&self) -> &Ext {
+        &self.ext
+    }
+}
+
 impl ExtKey {
     /// Set the default "Open With" of this file extension to given ProgId.
     pub fn link(&self, prog_id: &ProgIdKey, scope: Scope) -> Result<()> {
@@ -243,7 +271,7 @@ impl ExtKey {
         let (subkey, _) =
             classes.create_subkey_with_flags(blank_path_guard(self.ext.to_string())?, KEY_WRITE)?;
         // Set the default way to open this file extension
-        subkey.set_value("", &prog_id.to_string())?;
+        subkey.set_value("", &prog_id.as_inner().to_string())?;
 
         // Okey
         Ok(())
@@ -268,7 +296,7 @@ impl ExtKey {
         )? {
             // Only delete the default key if it is equal to our ProgId
             if let Some(value) = try_get_value::<String, _>(&subkey, "")? {
-                if value == prog_id.to_string() {
+                if value == prog_id.as_inner().to_string() {
                     // Delete the default key.
                     subkey.delete_value("")?;
                 }
@@ -297,9 +325,7 @@ impl ExtKey {
             Some(subkey) => {
                 // Try get associated ProgId if possible
                 match try_get_value::<String, _>(&subkey, "")? {
-                    Some(value) => {
-                        Some(ProgIdKey::from_str(value.as_str()).expect("unexpected Infallable"))
-                    }
+                    Some(value) => Some(ProgIdKey::new(value.as_str())?),
                     None => None,
                 }
             }
@@ -311,22 +337,6 @@ impl ExtKey {
     }
 }
 
-impl Display for ExtKey {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "{}", self.ext)
-    }
-}
-
-impl FromStr for ExtKey {
-    type Err = <Ext as FromStr>::Err;
-
-    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
-        Ok(Self {
-            ext: Ext::from_str(s)?,
-        })
-    }
-}
-
 // endregion
 
 // region: ProgId Registry Key
@@ -336,6 +346,20 @@ pub struct ProgIdKey {
     prog_id: ProgIdKind,
 }
 
+impl ProgIdKey {
+    /// Create new ProgId registry representer.
+    pub fn new(s: &str) -> Result<Self> {
+        Ok(Self {
+            prog_id: ProgIdKind::from_str(s)?,
+        })
+    }
+
+    /// Fetch the reference to inner ProgId.
+    pub fn as_inner(&self) -> &ProgIdKind {
+        &self.prog_id
+    }
+}
+
 impl ProgIdKey {
     /// Create ProgId into Registry in given scope with given parameters
     pub fn create(&self, scope: Scope, command: &str) -> Result<()> {
@@ -364,20 +388,4 @@ impl ProgIdKey {
     }
 }
 
-impl Display for ProgIdKey {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "{}", self.prog_id)
-    }
-}
-
-impl FromStr for ProgIdKey {
-    type Err = <ProgIdKind as FromStr>::Err;
-
-    fn from_str(s: &str) -> std::result::Result<Self, Self::Err> {
-        Ok(Self {
-            prog_id: ProgIdKind::from_str(s)?,
-        })
-    }
-}
-
 // endregion
diff --git a/wfassoc/src/extra/winreg.rs b/wfassoc/src/extra/winreg.rs
index 014cb84..f56df09 100644
--- a/wfassoc/src/extra/winreg.rs
+++ b/wfassoc/src/extra/winreg.rs
@@ -62,14 +62,14 @@ pub fn try_get_value<T: FromRegValue, N: AsRef<OsStr>>(
 
 // endregion
 
-// region: Blank String Guard
+// region: Blank Path Guard
 
-/// The error occurs when given string is empty.
+/// The error occurs when given registry path is empty.
 #[derive(Debug, TeError)]
-#[error("unexpected empty string")]
-pub struct BlankStringError {}
+#[error("unexpected empty registry path")]
+pub struct BlankPathError {}
 
-impl BlankStringError {
+impl BlankPathError {
     fn new() -> Self {
         Self {}
     }
@@ -82,9 +82,9 @@ impl BlankStringError {
 /// Because it will cause unexpected behavior that returning key self, rather than subkey.
 /// This is VERY dangerous especially for those registry delete functions.
 /// So I create this function to prevent any harmful blank path was passed into registry function.
-pub fn blank_path_guard<P: AsRef<OsStr>>(path: P) -> std::result::Result<P, BlankStringError> {
+pub fn blank_path_guard<P: AsRef<OsStr>>(path: P) -> std::result::Result<P, BlankPathError> {
     if path.as_ref().is_empty() {
-        Err(BlankStringError::new())
+        Err(BlankPathError::new())
     } else {
         Ok(path)
     }