diff --git a/wfassoc/src/highlevel.rs b/wfassoc/src/highlevel.rs
index f185c0e..3d94a97 100644
--- a/wfassoc/src/highlevel.rs
+++ b/wfassoc/src/highlevel.rs
@@ -1,8 +1,10 @@
 use crate::{lowlevel, utilities, win32::concept};
+use regex::Regex;
 use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::path::Path;
 use std::rc::Rc;
+use std::sync::LazyLock;
 use thiserror::Error as TeError;
 
 pub use lowlevel::{Scope, View};
@@ -19,6 +21,10 @@ pub enum SchemaError {
 /// Error occurs when trying converting `Schema` into `Program`.
 #[derive(Debug, TeError)]
 pub enum ParseProgramError {
+    #[error("{0}")]
+    BadExtBody(#[from] concept::BadExtBodyError),
+    #[error("{0}")]
+    BadProgIdPart(#[from] concept::BadProgIdPartError),
     #[error("{0}")]
     BadFileName(#[from] concept::BadFileNameError),
     #[error("{0}")]
@@ -31,6 +37,10 @@ pub enum ParseProgramError {
     NoDirNamePart,
     #[error("given identifier is not presented in dict")]
     NoSuchIdentifier,
+    #[error("extension name should not be empty")]
+    EmptyExtension,
+    #[error("given program identifier is not allowed")]
+    BadIdentifier,
 }
 
 /// Error occurs when operating with `Program`.
@@ -124,6 +134,9 @@ impl Schema {
         }
     }
 
+    ///
+    ///
+    /// The passed `ext` string should be without leading dot `.`.
     pub fn add_ext(
         &mut self,
         ext: &str,
@@ -241,14 +254,39 @@ impl Program {
         index_map: &HashMap<String, usize>,
     ) -> Result<Rc<T>, ParseProgramError> {
         match index_map.get(key) {
-            Some(index) => match vector.get(*index) {
-                Some(value) => Ok(value.clone()),
-                None => Err(ParseProgramError::NoSuchIdentifier),
-            },
+            Some(index) => Ok(vector
+                .get(*index)
+                .expect("unexpected invalid index")
+                .clone()),
             None => Err(ParseProgramError::NoSuchIdentifier),
         }
     }
 
+    /// Build ProgId from identifier and given file extension.
+    fn build_progid(
+        identifier: &str,
+        ext: &str,
+    ) -> Result<lowlevel::LosseProgId, ParseProgramError> {
+        // Use Regex to check identifier
+        static RE: LazyLock<Regex> = LazyLock::new(|| {
+            Regex::new(r"^[a-zA-Z][a-zA-Z0-9_-]*$").expect("unexpected bad regex pattern string")
+        });
+        let identifier = match RE.captures(identifier) {
+            Some(_) => identifier,
+            None => return Err(ParseProgramError::BadIdentifier),
+        };
+
+        // Capitalize first ASCII of ext
+        let ext = utilities::capitalize_first_ascii(ext);
+
+        // Build strict ProgId
+        let progid = concept::ProgId::new(identifier, &ext, None)?;
+        // Then build losse ProgId
+        let losse_progid: lowlevel::LosseProgId = progid.into();
+        // Return built result
+        Ok(losse_progid)
+    }
+
     /// Try creating Program from Schema.
     pub fn new(schema: Schema) -> Result<Self, ParseProgramError> {
         // Extract file name part and directory name part respectively.
@@ -301,16 +339,60 @@ impl Program {
             .map(|behavior| Self::resolve_index(&behavior, &behaviors, &behaviors_index_map))
             .transpose()?;
 
-        // Process file extensions.
-        // We need create ProgIds list and Exts list at the same time.
+        // We build ProgIdKey and ExtKey list at the same time
+        let mut progid_keys: Vec<Rc<ProgramProgIdKey>> = Vec::with_capacity(schema.exts.len());
+        let mut ext_keys: Vec<ProgramExtKey> = Vec::with_capacity(schema.exts.len());
+        for (key, value) in &schema.exts {
+            // Build ProgId first.
+            let progid = Self::build_progid(&schema.identifier, key.as_str())?;
+            // Then build ProgId key.
+            let progid_key = lowlevel::ProgIdKey::new(progid);
 
-        todo!();
-        // Ok(Self {
-        //     app_paths_key,
-        //     applications_key,
-        //     file_name,
-        //     dir_name,
-        // })
+            // Build essential fields for program ProgId key struct.
+            let name = Self::resolve_index(&value.name, &strs, &strs_index_map)?;
+            let icon = Self::resolve_index(&value.icon, &icons, &icons_index_map)?;
+            let behavior = Self::resolve_index(&value.behavior, &behaviors, &behaviors_index_map)?;
+
+            // Create program ProgId key struct
+            let progid_key = ProgramProgIdKey {
+                key: progid_key,
+                name,
+                icon,
+                behavior,
+            };
+            // Create Rc style
+            let progid_key = Rc::new(progid_key);
+
+            // Build Ext.
+            let ext = concept::Ext::new(key.as_str())?;
+            let ext_key = lowlevel::ExtKey::new(ext);
+
+            // Create program Ext key struct
+            let ext_key = ProgramExtKey {
+                key: ext_key,
+                assoc: progid_key.clone(),
+            };
+
+            // Add them into list
+            progid_keys.push(progid_key);
+            ext_keys.push(ext_key);
+        }
+
+        // Everything is okey
+        Ok(Self {
+            app_paths_key,
+            applications_key,
+            file_name,
+            dir_name,
+            name,
+            icon,
+            behavior,
+            strs,
+            icons,
+            behaviors,
+            progid_keys,
+            ext_keys,
+        })
     }
 }