Developer Guide

Last updated Oct 30, 2025

Writing the code for a custom activity

To better understand how to write the code for a custom activity, we are going to create a simple activity which asks the user for two numbers, then outputs the square of their sum.

Launch Microsoft Visual Studio.
Select File > New >Project… (shortcut: Ctrl + Shift + N). The New Project window is displayed.
Select C# from the Languages dropdown menu. The list of all dependencies using C# is displayed.
Select Class Library (.NET Framework). This helps us export the custom activity as a .dll file. Select Next to go to the Configure your new project window.
Fill in the Project name field with the desired activity name. In our case, we can use "MathSquareOfSum".
Select .NET Framework 4.6.1 from the Framework dropdown menu. This ensures that the library is compatible with UiPath Studio.

Important: In Windows - Legacy projects, UiPath Studio supports activities created with .NET Framework 4.5.2 to 4.6.1. For information on how to migrate activities to .NET for use in projects with the Windows compatibility, see Migrating Activities to .NET 6.
Select Create to go to the code designer and start writing the activity code.
In the Solution Explorer panel, right-click the References branch and select Add Reference.... The Reference Manager window is displayed.
Search for the System.Activities and System.ComponentModel.Composition references and add them. Make sure you check the box in front of each of them and click OK. Doing so enables us to use classes from the aforementioned references.
Now, we need to make sure that our code uses the newly-added references. This is done by adding the following lines in the code designer:
```
using System.Activities;
using System.ComponentModel;using System.Activities;
using System.ComponentModel;
```
Our project should now look like this:

Add the input and output parameters. In our case, the code should look like this:

//Note that these attributes are localized so you need to localize this attribute for Studio languages other than English
 
//Dots allow for hierarchy. App Integration.Excel is where Excel activities are.
[Category("Category.Where.Your.Activity.Appears.In.Toolbox")]
[DisplayName("Human readable name instead of class name")]
[Description("The text of the tooltip")]
 public class MathSqSum : CodeActivity
{
 
    //Note that these attributes are localized so you need to localize this attribute for Studio languages other than English
             [Category("Input")]
        [DisplayName("First Number")]
        [Description("Enter the first number")]
        [RequiredArgument]
        public InArgument<int> FirstNumber { get; set; }
        [Category("Input")]
        [DisplayName("Second Number")]
        [Description("Enter the second number")]
        [RequiredArgument]
        public InArgument<int> SecondNumber { get; set; }
        [Category("Output")]
        public OutArgument<int> ResultNumber { get; set; }
        protected override void Execute(CodeActivityContext context)
        {
        }//Note that these attributes are localized so you need to localize this attribute for Studio languages other than English
 
//Dots allow for hierarchy. App Integration.Excel is where Excel activities are.
[Category("Category.Where.Your.Activity.Appears.In.Toolbox")]
[DisplayName("Human readable name instead of class name")]
[Description("The text of the tooltip")]
 public class MathSqSum : CodeActivity
{
 
    //Note that these attributes are localized so you need to localize this attribute for Studio languages other than English
             [Category("Input")]
        [DisplayName("First Number")]
        [Description("Enter the first number")]
        [RequiredArgument]
        public InArgument<int> FirstNumber { get; set; }
        [Category("Input")]
        [DisplayName("Second Number")]
        [Description("Enter the second number")]
        [RequiredArgument]
        public InArgument<int> SecondNumber { get; set; }
        [Category("Output")]
        public OutArgument<int> ResultNumber { get; set; }
        protected override void Execute(CodeActivityContext context)
        {
        }

The <DisplayName(" ")> attribute is the label shown before the input field in the Properties panel in Studio. The <Description(" ")> attribute is the text of the tooltip displayed on mouse hover.

The <ReguiredArgument> attribute is needed if you want a declared element to be mandatory for input. If it is not filled in, a caution icon appears in the title bar of the activity.

Add the functionality in the <Execute( )> overridden function. In our case, it looks like this:

protected override void Execute(CodeActivityContext context)
{
    var firstNumber = FirstNumber.Get(context);
  var secondNumber = SecondNumber.Get(context);
  
  var result = (int)Math.Pow(firstNumber + secondNumber, 2);
  ResultNumber.Set(context, result);
}protected override void Execute(CodeActivityContext context)
{
    var firstNumber = FirstNumber.Get(context);
  var secondNumber = SecondNumber.Get(context);
  
  var result = (int)Math.Pow(firstNumber + secondNumber, 2);
  ResultNumber.Set(context, result);
}

(Optional) Building a Designer Interface

If you do not wish for your activity to have a designer interface, you can skip to the building the library section.

Important:

In order to build a Designer Interface, you need to have the Windows Workflow Foundation component installed in Visual Studio. If you have not selected the component from the Visual Studio installer, you can add it as follows:

In Visual Studio, click the Tools menu, and select Get Tools and Features.... The Visual Studio Installer window shows up.
Switch to the Individual Components tab and search for the Windows Workflow Foundation component. It is located under the Development activities section.
Check the box in front of the Windows Workflow Foundation component and click Modify. The required component gets installed.

Right-click the project from the Properties panel (in our case, the project is MathSquareOfSum). The context menu shows up.
From the Add item, select New Item.... The Add New Item window is displayed.
Select Workflow under the Installed category on the left panel. All related elements are displayed.
Select Activity Designer and click Add to include the item in the project.

The Activity Designer item is added, and the corresponding .xaml file is opened right away. It should look like this:

Replace the existing Activity Designer code with the following:

<sap:ActivityDesigner x:Class="MathSquareOfSum.MathSqSumDesigner"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:s="clr-namespace:System;assembly=mscorlib"
    xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
    xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation"
    xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation">
    <a href="sap:ActivityDesigner.Resources">sap:ActivityDesigner.Resources</a>
        <ResourceDictionary>
            <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter" />
        </ResourceDictionary>
    </sap:ActivityDesigner.Resources>
    <DockPanel Width="300">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="90"></ColumnDefinition>
                <ColumnDefinition Width="210"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition></RowDefinition>
                <RowDefinition></RowDefinition>
                <RowDefinition></RowDefinition>
            </Grid.RowDefinitions>
            <TextBlock Grid.Row="0" Grid.Column="0" Text="First Number"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="0" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="Enter first number" Expression="{Binding Path=ModelItem.FirstNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=In, Mode=TwoWay}" />
            <TextBlock Grid.Row="1" Grid.Column="0" Text="Second Number"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="1" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="Enter second number" Expression="{Binding Path=ModelItem.SecondNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=In, Mode=TwoWay}" />
            <TextBlock Grid.Row="2" Grid.Column="0" Text="Result"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="2" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="The sum of the numbers" UseLocationExpression="True" Expression="{Binding Path=ModelItem.ResultNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=Out, Mode=TwoWay}" />
        </Grid>
    </DockPanel>
</sap:ActivityDesigner><sap:ActivityDesigner x:Class="MathSquareOfSum.MathSqSumDesigner"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:s="clr-namespace:System;assembly=mscorlib"
    xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
    xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation"
    xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation">
    <a href="sap:ActivityDesigner.Resources">sap:ActivityDesigner.Resources</a>
        <ResourceDictionary>
            <sapc:ArgumentToExpressionConverter x:Key="ArgumentToExpressionConverter" />
        </ResourceDictionary>
    </sap:ActivityDesigner.Resources>
    <DockPanel Width="300">
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="90"></ColumnDefinition>
                <ColumnDefinition Width="210"></ColumnDefinition>
            </Grid.ColumnDefinitions>
            <Grid.RowDefinitions>
                <RowDefinition></RowDefinition>
                <RowDefinition></RowDefinition>
                <RowDefinition></RowDefinition>
            </Grid.RowDefinitions>
            <TextBlock Grid.Row="0" Grid.Column="0" Text="First Number"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="0" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="Enter first number" Expression="{Binding Path=ModelItem.FirstNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=In, Mode=TwoWay}" />
            <TextBlock Grid.Row="1" Grid.Column="0" Text="Second Number"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="1" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="Enter second number" Expression="{Binding Path=ModelItem.SecondNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=In, Mode=TwoWay}" />
            <TextBlock Grid.Row="2" Grid.Column="0" Text="Result"></TextBlock>
            <sapv:ExpressionTextBox Grid.Row="2" Grid.Column="1"  OwnerActivity="{Binding Path=ModelItem}" ExpressionType="s:Int32" HintText="The sum of the numbers" UseLocationExpression="True" Expression="{Binding Path=ModelItem.ResultNumber, Converter={StaticResource ArgumentToExpressionConverter},ConverterParameter=Out, Mode=TwoWay}" />
        </Grid>
    </DockPanel>
</sap:ActivityDesigner>

The new defined layout for the activity should now look like this:

Right-click the activity (in our case ActMathSquareOfSum), and from the Add menu, select Class.... The Add New Item window is displayed.
The Class item is already selected. All that is needed now is to rename it to DesignerMetadata.cs, and select Add. The new class is now added to the activity and opens up in a new tab.

Add the following content in the newly-created DesignerMetadata class:

using MathSquareOfSum;
using System.Activities.Presentation.Metadata;
using System.ComponentModel;
namespace ActMathSquareOfSum
{
    public class DesignerMetadata : IRegisterMetadata
    {
        public void Register()
        {
            AttributeTableBuilder attributeTableBuilder = new AttributeTableBuilder();
            attributeTableBuilder.AddCustomAttributes(typeof(MathSqSum), new DesignerAttribute(typeof(MathSqSumDesigner)));
            MetadataStore.AddAttributeTable(attributeTableBuilder.CreateTable());
        }
    }
}using MathSquareOfSum;
using System.Activities.Presentation.Metadata;
using System.ComponentModel;
namespace ActMathSquareOfSum
{
    public class DesignerMetadata : IRegisterMetadata
    {
        public void Register()
        {
            AttributeTableBuilder attributeTableBuilder = new AttributeTableBuilder();
            attributeTableBuilder.AddCustomAttributes(typeof(MathSqSum), new DesignerAttribute(typeof(MathSqSumDesigner)));
            MetadataStore.AddAttributeTable(attributeTableBuilder.CreateTable());
        }
    }
}

On this page

(Optional) Building a Designer Interface

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PREVIOUSCreating Activities With Code (Legacy)

NEXTAdding the external assembly .dll in UiPath Studio

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