diff --git a/.gitignore b/.gitignore
index 58d4acc..ff9ce1a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -402,4 +402,7 @@ ASALocalRun/
 .mfractor/
 
 # Local History for Visual Studio
-.localhistory/
\ No newline at end of file
+.localhistory/
+
+# Customization
+Svg2Png/
\ No newline at end of file
diff --git a/Billing.Shared/Billing.Shared.projitems b/Billing.Shared/Billing.Shared.projitems
index 55ce733..2f2dce2 100644
--- a/Billing.Shared/Billing.Shared.projitems
+++ b/Billing.Shared/Billing.Shared.projitems
@@ -21,6 +21,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)MainShell.xaml.cs">
       <DependentUpon>MainShell.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Models\BaseModel.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Models\Billing.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Models\Category.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Themes\BaseTheme.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Themes\Dark.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Themes\Light.cs" />
diff --git a/Billing.Shared/Models/BaseModel.cs b/Billing.Shared/Models/BaseModel.cs
new file mode 100644
index 0000000..647af34
--- /dev/null
+++ b/Billing.Shared/Models/BaseModel.cs
@@ -0,0 +1,182 @@
+using System;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Xml.Linq;
+
+namespace Billing.Models;
+
+public interface IModel
+{
+    void OnXmlSerialize(XElement node);
+
+    void OnXmlDeserialize(XElement node);
+}
+
+public abstract class BaseModel : IModel, IDisposable
+{
+    private bool disposed = false;
+
+    public static T ParseXml<T>(string xml) where T : BaseModel, new()
+    {
+        XDocument doc = XDocument.Parse(xml);
+        T model = new();
+        model.OnXmlDeserialize(doc.Root);
+        return model;
+    }
+
+    protected static string ToString(IFormattable v) => v.ToString(null, CultureInfo.InvariantCulture);
+    protected static double ParseDouble(string s) => double.Parse(s, CultureInfo.InvariantCulture);
+    protected static decimal ParseDecimal(string s) => decimal.Parse(s, CultureInfo.InvariantCulture);
+    protected static int ParseInt(string s) => int.Parse(s, CultureInfo.InvariantCulture);
+    protected static long ParseLong(string s) => long.Parse(s, CultureInfo.InvariantCulture);
+
+    protected static bool IsTrue(string s) =>
+        string.Equals(s, "true", StringComparison.OrdinalIgnoreCase) ||
+        string.Equals(s, "yes", StringComparison.OrdinalIgnoreCase) ||
+        s == "1";
+
+    #region Private Methods
+
+    private static XElement WriteString(XElement parent, string name, string val, Action<XElement> action = null)
+    {
+        XElement ele;
+        if (val == null)
+        {
+            ele = new XElement(name);
+        }
+        else
+        {
+            ele = new XElement(name, val);
+        }
+        action?.Invoke(ele);
+        parent.Add(ele);
+        return ele;
+    }
+
+    private static T ReadSubnode<T>(XElement node, string subname, Func<XElement, T> func)
+    {
+        var ele = node.Elements().FirstOrDefault(e => string.Equals(e.Name.ToString(), subname, StringComparison.OrdinalIgnoreCase));
+        return func(ele);
+    }
+
+    private static T ReadObject<T>(XElement node) where T : IModel, new()
+    {
+        if (IsTrue(node.Attribute("null")?.Value))
+        {
+            return default;
+        }
+        T value = new();
+        value.OnXmlDeserialize(node);
+        return value;
+    }
+
+    private static T[] ReadArray<T>(XElement node) where T : IModel, new()
+    {
+        if (IsTrue(node.Attribute("null")?.Value))
+        {
+            return default;
+        }
+        int count = ParseInt(node.Attribute("count").Value);
+        T[] array = new T[count];
+        foreach (XElement ele in node.Elements("item"))
+        {
+            int index = ParseInt(ele.Attribute("index").Value);
+            array[index] = ReadObject<T>(ele);
+        }
+        return array;
+    }
+
+    #endregion
+
+    #region Read/Write Helper
+
+    protected static XElement Write(XElement parent, string name, string val) => WriteString(parent, name, val);
+    protected static XElement Write(XElement parent, string name, DateTime date) => WriteString(parent, name, ToString(date.Ticks));
+    protected static XElement Write<T>(XElement parent, string name, T value) where T : IFormattable => WriteString(parent, name, ToString(value));
+    protected static XElement WriteObject<T>(XElement parent, string name, T value) where T : IModel => WriteString(parent, name, null,
+        action: ele =>
+        {
+            if (value == null)
+            {
+                ele.Add(new XAttribute("null", 1));
+            }
+            else
+            {
+                value.OnXmlSerialize(ele);
+            }
+        });
+    protected static XElement WriteArray<T>(XElement parent, string name, T[] value) where T : IModel => WriteString(parent, name, null,
+        action: ele =>
+        {
+            if (value == null)
+            {
+                ele.Add(new XAttribute("null", 1));
+            }
+            else
+            {
+                ele.Add(new XAttribute("count", value.Length));
+                for (var i = 0; i < value.Length; i++)
+                {
+                    XElement item = WriteObject(ele, "item", value[i]);
+                    item.Add(new XAttribute("index", i));
+                }
+            }
+        });
+
+    protected static string Read(XElement node, string subname, string def) => ReadSubnode(node, subname, e => e?.Value ?? def);
+    protected static int Read(XElement node, string subname, int def) => ReadSubnode(node, subname, e => e == null ? def : ParseInt(e.Value));
+    protected static long Read(XElement node, string subname, long def) => ReadSubnode(node, subname, e => e == null ? def : ParseLong(e.Value));
+    protected static double Read(XElement node, string subname, double def) => ReadSubnode(node, subname, e => e == null ? def : ParseDouble(e.Value));
+    protected static decimal Read(XElement node, string subname, decimal def) => ReadSubnode(node, subname, e => e == null ? def : ParseDecimal(e.Value));
+    protected static DateTime Read(XElement node, string subname, DateTime def) => ReadSubnode(node, subname, e => e == null ? def : new DateTime(ParseLong(e.Value)));
+    protected static T ReadObject<T>(XElement node, string subname, T def = default) where T : IModel, new() => ReadSubnode(node, subname, e => e == null ? def : ReadObject<T>(e));
+    protected static T[] ReadArray<T>(XElement node, string subname, T[] def = null) where T : IModel, new() => ReadSubnode(node, subname, e => e == null ? def : ReadArray<T>(e));
+
+    #endregion
+
+    public XDocument ToXml()
+    {
+        XDocument xdoc = new(new XDeclaration("1.0", "utf-8", "yes"), new XElement("root"));
+        ToXml(xdoc.Root);
+        return xdoc;
+    }
+
+    public void ToXml(XElement node)
+    {
+        OnXmlSerialize(node);
+    }
+
+    public void SaveToStream(Stream stream)
+    {
+        ToXml().Save(stream, SaveOptions.DisableFormatting);
+    }
+
+    public abstract void OnXmlSerialize(XElement node);
+    public abstract void OnXmlDeserialize(XElement node);
+
+
+    public override string ToString()
+    {
+        XDocument xdoc = ToXml();
+        using MemoryStream ms = new();
+        using StreamWriter writer = new(ms, Encoding.UTF8);
+        xdoc.Save(writer, SaveOptions.DisableFormatting);
+        writer.Flush();
+        ms.Seek(0, SeekOrigin.Begin);
+        using StreamReader reader = new(ms, Encoding.UTF8);
+        return reader.ReadToEnd();
+    }
+
+    protected virtual void Dispose(bool dispose) { }
+
+    public void Dispose()
+    {
+        if (!disposed)
+        {
+            Dispose(true);
+            disposed = true;
+        }
+    }
+}
diff --git a/Billing.Shared/Models/Billing.cs b/Billing.Shared/Models/Billing.cs
new file mode 100644
index 0000000..0b38869
--- /dev/null
+++ b/Billing.Shared/Models/Billing.cs
@@ -0,0 +1,31 @@
+using System;
+using System.Xml.Linq;
+
+namespace Billing.Models;
+
+public class Billing : BaseModel
+{
+    public decimal Amount { get; set; }
+    public string Name { get; set; }
+    public int CategoryId { get; set; }
+    public string Store { get; set; }
+    public DateTime CreateTime { get; set; }
+
+    public override void OnXmlDeserialize(XElement node)
+    {
+        Amount = Read(node, nameof(Amount), 0m);
+        Name = Read(node, nameof(Name), string.Empty);
+        CategoryId = Read(node, nameof(CategoryId), -1);
+        Store = Read(node, nameof(Store), string.Empty);
+        CreateTime = Read(node, nameof(CreateTime), default(DateTime));
+    }
+
+    public override void OnXmlSerialize(XElement node)
+    {
+        Write(node, nameof(Amount), Amount);
+        Write(node, nameof(Name), Name);
+        Write(node, nameof(CategoryId), CategoryId);
+        Write(node, nameof(Store), Store);
+        Write(node, nameof(CreateTime), CreateTime);
+    }
+}
diff --git a/Billing.Shared/Models/Category.cs b/Billing.Shared/Models/Category.cs
new file mode 100644
index 0000000..213aa1b
--- /dev/null
+++ b/Billing.Shared/Models/Category.cs
@@ -0,0 +1,31 @@
+using System.Xml.Linq;
+
+namespace Billing.Models;
+
+public class Category : BaseModel
+{
+    public int Id { get; set; }
+    public string Name { get; set; }
+    public int? ParentId { get; set; }
+
+    public override void OnXmlDeserialize(XElement node)
+    {
+        Id = Read(node, nameof(Id), 0);
+        Name = Read(node, nameof(Name), string.Empty);
+        var parentId = Read(node, nameof(ParentId), -1);
+        if (parentId >= 0)
+        {
+            ParentId = parentId;
+        }
+    }
+
+    public override void OnXmlSerialize(XElement node)
+    {
+        Write(node, nameof(Id), Id);
+        Write(node, nameof(Name), Name);
+        if (ParentId != null)
+        {
+            Write(node, nameof(ParentId), ParentId.Value);
+        }
+    }
+}
diff --git a/Billing/Billing.iOS/Billing.iOS.csproj b/Billing/Billing.iOS/Billing.iOS.csproj
index dfecd7b..5fd347b 100644
--- a/Billing/Billing.iOS/Billing.iOS.csproj
+++ b/Billing/Billing.iOS/Billing.iOS.csproj
@@ -130,6 +130,7 @@
     <Reference Include="System" />
     <Reference Include="System.Xml" />
     <Reference Include="System.Core" />
+    <Reference Include="System.Xml.Linq" />
     <Reference Include="Xamarin.iOS" />
     <Reference Include="System.Numerics" />
     <Reference Include="System.Numerics.Vectors" />