Custom Functions in the Expression Editor (ASP.NET Web Forms)

• Jul 08, 2021
• 3 minutes to read

This topic describes how to add a custom function to the Expression Editor or remove a function from the list of available functions.

Add a Custom Function

Implement a Custom Function

To create a custom function, create an object that descends from the ReportCustomFunctionOperatorBase abstract class.

The following code defines a custom function:

CustomFormatFunction.cs

using DevExpress.XtraReports.Expressions;
using System;
// ...
    public class CustomFormatFunction : ReportCustomFunctionOperatorBase
    {
        public override string FunctionCategory 
            => "String";
        public override string Description 
            => "CustomFormatFunction(string format, object arg0)" +
            "\r\nConverts an arg0 value to a string based on a specified format";
        public override bool IsValidOperandCount(int count) 
            => count == 2;
        public override bool IsValidOperandType(int operandIndex, int operandCount, Type type) 
            => true;
        public override int MaxOperandCount
            => 2;
        public override int MinOperandCount
            => 2;
        public override object Evaluate(params object[] operands)
        {
            string res = String.Format(operands[0].ToString(), operands[1]);
            return res;
        }
        public override string Name 
            => "CustomFormatFunction";
        public override Type ResultType(params Type[] operands)
        {
            return typeof(string);
        }
    }

Register a Function on the Server

Call the CustomFunctions.Register method at application startup:

Startup.cs

public class Startup {
    //...
    public void ConfigureServices(IServiceCollection services) {
        // ...
        services.ConfigureReportingServices(configurator => {
            configurator.ConfigureReportDesigner(designerConfigurator => {
                //...
                DevExpress.XtraReports.Expressions.CustomFunctions.Register(new CustomFormatFunction());
            });
            //...
        });
        //...
    }
    //...
}

Register a Function on the Client

Handle the BeforeRender event and use the following code to add the CustomFormatFunction to the list of available functions in the Expression Editor:

function OnBeforeRender(event) {
    DevExpress.Analytics.Localization.loadMessages({
        "CustomStringId.CustomFormatFunction":
            "CustomFormatFunction(string format, object arg0)\r\n" +
        "Converts an arg0 value to a string based on the specified format."

    });
    DevExpress.Reporting.Designer.Widgets.reportFunctionDisplay.push({
        display: "Custom",
        items: {
            // The function (item) name is the CustomFormatFunction.Name property value. 
            CustomFormatFunction: [{
                paramCount: 2,
                text: "CustomFormatFunction('{0:C}', 0)",
                displayName: "CustomFormatFunction(format, arg0)",
                descriptionStringId: "CustomStringId.CustomFormatFunction"
            }]
        }
    });
}

Note

The code uses the ReportCustomFunctionOperatorBase.Name property value as the name of the function to register.

Result

The following image shows a custom function (CustomFormatFunction) in the list of available functions in the Expression Editor:

Remove a Function from the List of Available Functions

Handle the BeforeRender event and remove a function from the DevExpress.Reporting.Designer.Widgets.reportFunctionDisplay collection. To get access to a function, use the following notation:

DevExpress.Reporting.Designer.Widgets.reportFunctionDisplay[index].items[“function_name”]

where index is a function category index.

The following code removes the LocalDateTimeThisYear function from the DateTime category:

function OnBeforeRender(event) {
    delete DevExpress.Reporting.Designer.Widgets.reportFunctionDisplay[1].items["LocalDateTimeThisYear"];
}

Unregister a Function

If you remove a function from the DevExpress.Reporting.Designer.Widgets.reportFunctionDisplay collection on the client, the function is still registered and expressions with that function remain valid and can be evaluated. You can type the function name and compose a new expression manually.

If you have registered a custom function, you can call the CustomFunctions.Unregister method at application startup to unregister it:

Global.asax.cs

void Application_Start(object sender, EventArgs e) {
    // ...
    DevExpress.XtraReports.Expressions.CustomFunctions.Unregister("CustomFormatFunction");
}

After this code is executed, the CustomFormatFunction function is not available in the Expression Editor and won’t be evaluated in expressions.