diff --git a/wfassoc/src/assoc.rs b/wfassoc/src/assoc.rs
index da2adcf..07b0784 100644
--- a/wfassoc/src/assoc.rs
+++ b/wfassoc/src/assoc.rs
@@ -236,11 +236,3 @@ impl Display for Clsid {
 }
 
 // endregion
-
-// region: Icon Resource
-
-// endregion
-
-// region: String Resource
-
-// endregion
diff --git a/wfassoc/src/extra/windows.rs b/wfassoc/src/extra/windows.rs
index 238563e..da7d97b 100644
--- a/wfassoc/src/extra/windows.rs
+++ b/wfassoc/src/extra/windows.rs
@@ -9,9 +9,383 @@ use std::path::Path;
 use std::str::FromStr;
 use std::sync::LazyLock;
 use thiserror::Error as TeError;
+use uuid::Uuid;
 use widestring::{WideCString, WideChar, WideStr};
 use windows_sys::Win32::UI::WindowsAndMessaging::HICON;
 
+// region: File Extension
+
+/// The error occurs when constructing Ext with bad body.
+#[derive(Debug, TeError)]
+#[error("given file extension body \"{inner}\" is invalid")]
+pub struct BadExtBodyError {
+    /// The clone of string which is not a valid file extension body.
+    inner: String,
+}
+
+impl BadExtBodyError {
+    /// Create new error instance.
+    fn new(inner: &str) -> Self {
+        Self {
+            inner: inner.to_string(),
+        }
+    }
+}
+
+/// The struct representing an file extension which must start with dot (`.`)
+/// and followed by at least one arbitrary characters.
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Ext {
+    /// The body of file extension (excluding dot).
+    body: String,
+}
+
+impl Ext {
+    /// Create an new file extension.
+    ///
+    /// `body` is the body of file extension (excluding dot).
+    /// If you want to create this struct with ordinary extension string like `.jpg`,
+    /// please use `from_str()` instead.
+    pub fn new(body: &str) -> Result<Self, BadExtBodyError> {
+        // Check whether given body has dot or empty
+        if body.is_empty() || body.contains('.') {
+            Err(BadExtBodyError::new(body))
+        } else {
+            Ok(Self {
+                body: body.to_string(),
+            })
+        }
+    }
+
+    /// Get the body part of file extension (excluding dot)
+    pub fn inner(&self) -> &str {
+        &self.body
+    }
+}
+
+/// The error occurs when try parsing string into FileExt.
+#[derive(Debug, TeError)]
+#[error("given file extension name \"{inner}\" is invalid")]
+pub struct ParseExtError {
+    /// The clone of string which is not a valid file extension.
+    inner: String,
+}
+
+impl ParseExtError {
+    /// Create new error instance.
+    fn new(inner: &str) -> Self {
+        Self {
+            inner: inner.to_string(),
+        }
+    }
+}
+
+impl Display for Ext {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, ".{}", self.body)
+    }
+}
+
+impl FromStr for Ext {
+    type Err = ParseExtError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        static RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"^\.([^\.]+)$").unwrap());
+        match RE.captures(s) {
+            Some(v) => Ok(Self::new(&v[1]).expect("unexpected dot in Ext body")),
+            None => Err(ParseExtError::new(s)),
+        }
+    }
+}
+
+// endregion
+
+// region: Programmatic Identifiers (ProgId)
+
+/// The error occurs when constructing ProgId.
+#[derive(Debug, TeError)]
+#[error("given ProgId part \"{inner}\" is invalid")]
+pub struct BadProgIdPartError {
+    /// The clone of string which is not a valid ProgId part.
+    inner: String,
+}
+
+impl BadProgIdPartError {
+    /// Create new error instance.
+    fn new(s: &str) -> Self {
+        Self {
+            inner: s.to_string(),
+        }
+    }
+}
+
+/// The ProgId exactly follows Microsoft suggested
+/// `[Vendor or Application].[Component].[Version]` format.
+///
+/// However, most of applications do no follow this standard,
+/// this scenario is not convered by this struct in there.
+/// It should be done in other place.
+/// Additionally, `[Version]` part is optional.
+///
+/// Reference:
+/// - https://learn.microsoft.com/en-us/windows/win32/shell/fa-progids
+/// - https://learn.microsoft.com/en-us/windows/win32/com/-progid--key
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ProgId {
+    /// The vendor part of ProgId.
+    vendor: String,
+    /// The component part of ProgId.
+    component: String,
+    /// The optional version part of ProgId.
+    version: Option<u32>,
+}
+
+impl ProgId {
+    /// Create a new ProgId with given parts.
+    pub fn new(
+        vendor: &str,
+        component: &str,
+        version: Option<u32>,
+    ) -> Result<Self, BadProgIdPartError> {
+        // Check whether vendor or component part is empty or has dot
+        if vendor.is_empty() || vendor.contains('.') {
+            Err(BadProgIdPartError::new(vendor))
+        } else if component.is_empty() || component.contains('.') {
+            Err(BadProgIdPartError::new(component))
+        } else {
+            Ok(Self {
+                vendor: vendor.to_string(),
+                component: component.to_string(),
+                version,
+            })
+        }
+    }
+
+    /// Get the vendor part of standard ProgId.
+    pub fn get_vendor(&self) -> &str {
+        &self.vendor
+    }
+
+    /// Get the component part of standard ProgId.
+    pub fn get_component(&self) -> &str {
+        &self.component
+    }
+
+    /// Get the version part of standard ProgId.
+    pub fn get_version(&self) -> Option<u32> {
+        self.version
+    }
+}
+
+/// The error occurs when parsing ProgId.
+#[derive(Debug, TeError)]
+#[error("given ProgId \"{inner}\" is invalid")]
+pub struct ParseProgIdError {
+    /// The clone of string which is not a valid ProgId.
+    inner: String,
+}
+
+impl ParseProgIdError {
+    /// Create new error instance.
+    fn new(s: &str) -> Self {
+        Self {
+            inner: s.to_string(),
+        }
+    }
+}
+
+impl Display for ProgId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match &self.version {
+            Some(version) => write!(f, "{}.{}.{}", self.vendor, self.component, version),
+            None => write!(f, "{}.{}", self.vendor, self.component),
+        }
+    }
+}
+
+impl FromStr for ProgId {
+    type Err = ParseProgIdError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        static RE: LazyLock<Regex> =
+            LazyLock::new(|| Regex::new(r"^([^\.]+)\.([^\.]+)(\.([0-9]+))?$").unwrap());
+        let caps = RE.captures(s);
+        if let Some(caps) = caps {
+            let vendor = &caps[1];
+            let component = &caps[2];
+            let version = match caps.get(4) {
+                Some(sv) => Some(
+                    sv.as_str()
+                        .parse::<u32>()
+                        .map_err(|_| ParseProgIdError::new(s))?,
+                ),
+                None => None,
+            };
+            Ok(Self::new(vendor, component, version).expect("unexpected bad part of ProgId"))
+        } else {
+            Err(ParseProgIdError::new(s))
+        }
+    }
+}
+
+// endregion
+
+// region: CLSID
+
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Clsid {
+    inner: Uuid,
+}
+
+impl Clsid {
+    pub fn new(uuid: &str) -> Result<Self, ParseClsidError> {
+        Self::from_str(uuid)
+    }
+
+    // TODO: May add CLSID generator in there.
+}
+
+/// The error occurs when parsing CLSID
+#[derive(Debug, TeError)]
+#[error("given string \"{inner}\" is invalid for uuid")]
+pub struct ParseClsidError {
+    inner: String,
+}
+
+impl ParseClsidError {
+    fn new(s: &str) -> Self {
+        Self {
+            inner: s.to_string(),
+        }
+    }
+}
+
+impl FromStr for Clsid {
+    type Err = ParseClsidError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Self {
+            inner: Uuid::parse_str(s).map_err(|_| ParseClsidError::new(s))?,
+        })
+    }
+}
+
+impl Display for Clsid {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.inner.braced().to_string())
+    }
+}
+
+// endregion
+
+// region: Expand String
+
+/// Error occurs when creating Expand String.
+#[derive(Debug, TeError)]
+#[error("given string is not an expand string")]
+pub struct ParseExpandStrError {}
+
+impl ParseExpandStrError {
+    fn new() -> Self {
+        Self {}
+    }
+}
+
+/// Error occurs when expand Expand String
+#[derive(Debug, TeError)]
+#[error("error occurs when expanding expand string")]
+pub enum ExpandEnvVarError {
+    /// Given string has embedded NUL.
+    EmbeddedNul(#[from] widestring::error::ContainsNul<WideChar>),
+    /// The encoding of string is invalid.
+    BadEncoding(#[from] widestring::error::Utf16Error),
+    /// Error occurs when int type casting.
+    BadIntCast(#[from] std::num::TryFromIntError),
+    /// Integeral arithmatic downflow.
+    Underflow,
+    /// Error occurs when executing Win32 expand function.
+    ExpandFunction,
+    /// Some environment vairable are not expanded.
+    NoEnvVar,
+}
+
+/// The struct representing an Expand String,
+/// which contain environment variable in string,
+/// like `%LOCALAPPDATA%\SomeApp.exe`.
+pub struct ExpandString {
+    inner: String,
+}
+
+impl ExpandString {
+    const VAR_RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"%[a-zA-Z0-9_]+%").unwrap());
+}
+
+impl ExpandString {
+    /// Create a new expand string
+    pub fn new(s: &str) -> Result<Self, ParseExpandStrError> {
+        Self::from_str(s)
+    }
+
+    /// Expand the variables located in this string
+    /// and produce the final usable string.
+    pub fn expand_string(&self) -> Result<String, ExpandEnvVarError> {
+        use windows_sys::Win32::System::Environment::ExpandEnvironmentStringsW;
+
+        // Fetch the size of expand result
+        let source = WideCString::from_str(self.inner.as_str())?;
+        let size = unsafe { ExpandEnvironmentStringsW(source.as_ptr(), std::ptr::null_mut(), 0) };
+        if size == 0 {
+            return Err(ExpandEnvVarError::ExpandFunction);
+        }
+        let size_no_nul = size.checked_sub(1).ok_or(ExpandEnvVarError::Underflow)?;
+
+        // Allocate buffer for it.
+        let len: usize = size.try_into()?;
+        let len_no_nul = len.checked_sub(1).ok_or(ExpandEnvVarError::Underflow)?;
+        let mut buffer = vec![0; len];
+        // Receive result
+        let size =
+            unsafe { ExpandEnvironmentStringsW(source.as_ptr(), buffer.as_mut_ptr(), size_no_nul) };
+        if size == 0 {
+            return Err(ExpandEnvVarError::ExpandFunction);
+        }
+
+        // Cast result as Rust string
+        let wstr = unsafe { WideStr::from_ptr(buffer.as_ptr(), len_no_nul) };
+        let rv = wstr.to_string()?;
+
+        // If the final string still has environment variable,
+        // we think we fail to expand it.
+        if Self::VAR_RE.is_match(rv.as_str()) {
+            Err(ExpandEnvVarError::NoEnvVar)
+        } else {
+            Ok(rv)
+        }
+    }
+}
+
+impl Display for ExpandString {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.inner)
+    }
+}
+
+impl FromStr for ExpandString {
+    type Err = ParseExpandStrError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        if Self::VAR_RE.is_match(s) {
+            Ok(Self {
+                inner: s.to_string(),
+            })
+        } else {
+            Err(ParseExpandStrError::new())
+        }
+    }
+}
+
+// endregion
+
 // region: Windows Resource Reference String
 
 // region: Icon Reference String
@@ -359,114 +733,6 @@ impl StrRc {
 
 // endregion
 
-// region: Expand String
-
-/// Error occurs when creating Expand String.
-#[derive(Debug, TeError)]
-#[error("given string is not an expand string")]
-pub struct ParseExpandStrError {}
-
-impl ParseExpandStrError {
-    fn new() -> Self {
-        Self {}
-    }
-}
-
-/// Error occurs when expand Expand String
-#[derive(Debug, TeError)]
-#[error("error occurs when expanding expand string")]
-pub enum ExpandEnvVarError {
-    /// Given string has embedded NUL.
-    EmbeddedNul(#[from] widestring::error::ContainsNul<WideChar>),
-    /// The encoding of string is invalid.
-    BadEncoding(#[from] widestring::error::Utf16Error),
-    /// Error occurs when int type casting.
-    BadIntCast(#[from] std::num::TryFromIntError),
-    /// Integeral arithmatic downflow.
-    Underflow,
-    /// Error occurs when executing Win32 expand function.
-    ExpandFunction,
-    /// Some environment vairable are not expanded.
-    NoEnvVar,
-}
-
-/// The struct representing an Expand String,
-/// which contain environment variable in string,
-/// like `%LOCALAPPDATA%\SomeApp.exe`.
-pub struct ExpandString {
-    inner: String,
-}
-
-impl ExpandString {
-    const VAR_RE: LazyLock<Regex> = LazyLock::new(|| Regex::new(r"%[a-zA-Z0-9_]+%").unwrap());
-}
-
-impl ExpandString {
-    /// Create a new expand string
-    pub fn new(s: &str) -> Result<Self, ParseExpandStrError> {
-        Self::from_str(s)
-    }
-
-    /// Expand the variables located in this string
-    /// and produce the final usable string.
-    pub fn expand_string(&self) -> Result<String, ExpandEnvVarError> {
-        use windows_sys::Win32::System::Environment::ExpandEnvironmentStringsW;
-
-        // Fetch the size of expand result
-        let source = WideCString::from_str(self.inner.as_str())?;
-        let size = unsafe { ExpandEnvironmentStringsW(source.as_ptr(), std::ptr::null_mut(), 0) };
-        if size == 0 {
-            return Err(ExpandEnvVarError::ExpandFunction);
-        }
-        let size_no_nul = size.checked_sub(1).ok_or(ExpandEnvVarError::Underflow)?;
-
-        // Allocate buffer for it.
-        let len: usize = size.try_into()?;
-        let len_no_nul = len.checked_sub(1).ok_or(ExpandEnvVarError::Underflow)?;
-        let mut buffer = vec![0; len];
-        // Receive result
-        let size =
-            unsafe { ExpandEnvironmentStringsW(source.as_ptr(), buffer.as_mut_ptr(), size_no_nul) };
-        if size == 0 {
-            return Err(ExpandEnvVarError::ExpandFunction);
-        }
-
-        // Cast result as Rust string
-        let wstr = unsafe { WideStr::from_ptr(buffer.as_ptr(), len_no_nul) };
-        let rv = wstr.to_string()?;
-
-        // If the final string still has environment variable,
-        // we think we fail to expand it.
-        if Self::VAR_RE.is_match(rv.as_str()) {
-            Err(ExpandEnvVarError::NoEnvVar)
-        } else {
-            Ok(rv)
-        }
-    }
-}
-
-impl Display for ExpandString {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        write!(f, "{}", self.inner)
-    }
-}
-
-impl FromStr for ExpandString {
-    type Err = ParseExpandStrError;
-
-    fn from_str(s: &str) -> Result<Self, Self::Err> {
-        if Self::VAR_RE.is_match(s) {
-            Ok(Self {
-                inner: s.to_string(),
-            })
-        } else {
-            Err(ParseExpandStrError::new())
-        }
-    }
-}
-
-// endregion
-
 // region: Windows Commandline
 
 // region: Cmd Lexer
diff --git a/wfassoc/tests/extra_windows.rs b/wfassoc/tests/extra_windows.rs
index 032623e..ee02400 100644
--- a/wfassoc/tests/extra_windows.rs
+++ b/wfassoc/tests/extra_windows.rs
@@ -1,6 +1,90 @@
-use std::path::Path;
+use std::{path::Path, str::FromStr};
 use wfassoc::extra::windows::*;
 
+#[test]
+fn test_ex_new() {
+    fn ok_tester(s: &str, probe: &str) {
+        let rv = Ext::new(s);
+        assert!(rv.is_ok());
+        let rv = rv.unwrap();
+        assert_eq!(rv.to_string(), probe);
+    }
+    fn err_tester(s: &str) {
+        let rv = Ext::new(s);
+        assert!(rv.is_err());
+    }
+
+    ok_tester("jpg", ".jpg");
+    err_tester(".jpg");
+    err_tester("");
+}
+
+#[test]
+fn test_ext_parse() {
+    fn ok_tester(s: &str, probe: &str) {
+        let rv = Ext::from_str(s);
+        assert!(rv.is_ok());
+        let rv = rv.unwrap();
+        assert_eq!(rv.inner(), probe);
+    }
+    fn err_tester(s: &str) {
+        let rv = Ext::from_str(s);
+        assert!(rv.is_err());
+    }
+
+    ok_tester(".jpg", "jpg");
+    err_tester(".jar.disabled");
+    err_tester("jar");
+}
+
+#[test]
+fn test_prog_id_new() {
+    fn ok_tester(vendor: &str, component: &str, version: Option<u32>, probe: &str) {
+        let rv = ProgId::new(vendor, component, version);
+        assert!(rv.is_ok());
+        let rv = rv.unwrap();
+        assert_eq!(rv.to_string(), probe);
+    }
+    fn err_tester(vendor: &str, component: &str, version: Option<u32>) {
+        let rv = ProgId::new(vendor, component, version);
+        assert!(rv.is_err());
+    }
+
+    ok_tester("PowerPoint", "Template", Some(12), "PowerPoint.Template.12");
+    err_tester("", "MyApp", None);
+    err_tester("Me", "", None);
+    err_tester("M.e", "MyApp", None);
+    err_tester("Me", "My.App", None);
+}
+
+#[test]
+fn test_prog_id_parse() {
+    fn ok_tester(s: &str, probe_vendor: &str, probe_component: &str, probe_version: Option<u32>) {
+        let rv = ProgId::from_str(s);
+        assert!(rv.is_ok());
+        let rv =rv.unwrap();
+        assert_eq!(rv.get_vendor(), probe_vendor);
+        assert_eq!(rv.get_component(), probe_component);
+        assert_eq!(rv.get_version(), probe_version);
+    }
+    fn err_tester(s: &str) {
+        let rv = ProgId::from_str(s);
+        assert!(rv.is_err());
+    }
+
+    ok_tester("VSCode.c++", "VSCode", "c++", None);
+    ok_tester("PowerPoint.Template.12", "PowerPoint", "Template", Some(12));
+    err_tester("Me.MyApp.");
+    err_tester("WMP11.AssocFile.3G2");
+    err_tester("What the f*ck?");
+}
+
+#[test]
+fn test_clsid() {
+    fn ok_tester(s: &str) {}
+    fn err_tester(s: &str) {}
+}
+
 #[test]
 fn test_icon_ref_str() {
     fn ok_tester(s: &str, probe: (&str, u32)) {
@@ -15,7 +99,10 @@ fn test_icon_ref_str() {
         assert!(rv.is_err());
     }
 
-    ok_tester(r#"%SystemRoot%\System32\imageres.dll,-72"#, (r#"%SystemRoot%\System32\imageres.dll"#, 72));
+    ok_tester(
+        r#"%SystemRoot%\System32\imageres.dll,-72"#,
+        (r#"%SystemRoot%\System32\imageres.dll"#, 72),
+    );
     err_tester(r#"C:\Windows\Cursors\aero_arrow.cur"#);
     err_tester(r#"@%SystemRoot%\System32\shell32.dll,-30596"#);
 }
@@ -34,7 +121,10 @@ fn test_str_ref_str() {
         assert!(rv.is_err());
     }
 
-    ok_tester(r#"@%SystemRoot%\System32\shell32.dll,-30596"#, (r#"%SystemRoot%\System32\shell32.dll"#, 30596));
+    ok_tester(
+        r#"@%SystemRoot%\System32\shell32.dll,-30596"#,
+        (r#"%SystemRoot%\System32\shell32.dll"#, 30596),
+    );
     err_tester(r#"This is my application, OK?"#);
     err_tester(r#"%SystemRoot%\System32\imageres.dll,-72"#);
 }