Custom Aggregate Functions

The Telerik UI for WPF suite comes with its own data engine that is used in the RadGridView control. The engine provides a set of predefined aggregate functions that can be applied over the data - count, sum, max, average, etc.

This article describes how to implement custom aggregate functions by inheriting the EnumerableAggregateFunction EnumerableSelectorAggregateFunction classes, and also the generic AggregateFunction<TElement, TResult> class.

Implementing Custom EnumerableAggregateFunction

The EnumerableAggregateFunction represents a function that uses aggregate extension methods to return a specific result. Examples of Telerik built-in functions using this class are - CountFunction, FirstFunction and LastFunction.

To implement your own aggregate with custom behavior, inherit the EnumerableAggregateFunction and override its AggregateMethodName and ExtensionMethodsType members.

The ExtensionMethodsType tells the type of the static class that holds the methods used for aggregation.

The AggregateMethodName returns the name of the aggregation method from the static class.

Implementing a static class that holds the aggregation functions

• C#

public static class ClubModelAggregates 
{ 
    public static string SumGoalsPerHalfSeason<TSource>(IEnumerable<Club> clubs) 
    { 
        StringBuilder sb = new StringBuilder(); 
        double sum0 = 0, sum1 = 0; 
 
        foreach (var club in clubs) 
        { 
            if (club.Period != null ) 
            { 
                sum0 += club.FirstHalfSeasonNumberOfGoals; 
                sum1 += club.SecondHalfSeasonNumberOfGoals; 
            } 
        } 
 
        sb.Append("1st Half Season Total Goals: "); 
        sb.Append(sum0); 
        sb.Append(" "); 
        sb.AppendLine(); 
        sb.Append("2nd Half Season Total Goals: "); 
        sb.Append(sum1); 
 
        return sb.ToString(); 
    } 
} 

Implementing a custom function that calculates the median of the items

• C#

public class CustomSumFunction : EnumerableAggregateFunction 
{ 
    protected override string AggregateMethodName 
    { 
        get { return nameof(ClubModelAggregates.SumGoalsPerHalfSeason); } 
    } 
 
    protected override Type ExtensionMethodsType 
    { 
        get 
        { 
            return typeof(ClubModelAggregates); 
        } 
    } 
} 

The Club model used to populate the ItemsSource of RadGridView

• C#

public class Club : ViewModelBase 
{    
    // other properties here 
 
    private double firstHalfSeasonNumberOfGoals; 
    private double secondHalfSeasonNumberOfGoals; 
 
    public double FirstHalfSeasonNumberOfGoals  
    { 
        get { return this.firstHalfSeasonNumberOfGoals;  } 
        set { this.firstHalfSeasonNumberOfGoals = value; this.OnPropertyChanged(nameof(FirstHalfSeasonNumberOfGoals)); } 
    } 
 
    public double SecondHalfSeasonNumberOfGoals  
    { 
        get { return this.secondHalfSeasonNumberOfGoals;  } 
        set { this.secondHalfSeasonNumberOfGoals = value; this.OnPropertyChanged(nameof(SecondHalfSeasonNumberOfGoals)); } 
    } 
 
    public static ObservableCollection<Club> GetClubs() 
    { 
        return new ObservableCollection<Club>() 
        { 
            new Club() { FirstHalfSeasonNumberOfGoals = 44, SecondHalfSeasonNumberOfGoals = 39, /* other settings here / }, 
            new Club() { FirstHalfSeasonNumberOfGoals = 52, SecondHalfSeasonNumberOfGoals = 47, / other settings here / }, 
            new Club() { FirstHalfSeasonNumberOfGoals = 29, SecondHalfSeasonNumberOfGoals = 51, / other settings here / }, 
            new Club() { FirstHalfSeasonNumberOfGoals = 33, SecondHalfSeasonNumberOfGoals = 49, / other settings here */ }, 
        }; 
    } 
} 

Adding the function in the AggregateFunctions collection of RadGridView's columns

• XAML

<telerik:GridViewDataColumn DataMemberBinding="{Binding MyProperty}"> 
    <telerik:GridViewDataColumn.AggregateFunctions> 
        <local:CustomSumFunction /> 
    </telerik:GridViewDataColumn.AggregateFunctions> 
</telerik:GridViewDataColumn> 

Find a runnable project showing this approach in the following SDK sample.

Implementing Custom EnumerableSelectorAggregateFunction

The EnumerableSelectorAggregateFunction represents a function that uses a specific field from the data items in order to apply calculations over the data set. Examples of Telerik built-in functions using this class are - AverageFunction, MaxFunction, MinFunction, SumFunction.

To implement your own aggregate with custom behavior, inherit the EnumerableSelectorAggregateFunction and override its AggregateMethodName and ExtensionMethodsType members.

The ExtensionMethodsType tells the type of the static class that holds the methods used for aggregation.

The AggregateMethodName returns the name of the aggregation method from the static class.

Implementing a static class that holds the aggregation functions

• C#

public static class MedianAggregates 
{ 
    // Invoked when the SourceFiled of the AggregateFunction points to a property which Type is System.Int32 
    public static object GetMedian<TSource>(IEnumerable<TSource> items, Func<TSource, int> selector) 
    { 
        IEnumerable<int> selectedValues = items.Select(selector).Cast<int>(); 
        return GetMedian(selectedValues.ToArray()); 
    } 
 
    // Invoked when the SourceFiled of the AggregateFunction points to a property which Type is System.Double 
    public static object GetMedian<TSource>(IEnumerable<TSource> items, Func<TSource, double> selector) 
    { 
        IEnumerable<double> selectedValues = items.Select(selector).Cast<double>(); 
        return GetMedian(selectedValues.ToArray()); 
    } 
 
    // Invoked when the SourceFiled of the AggregateFunction points to a property which Type is System.Decimal 
    public static object GetMedian<TSource>(IEnumerable<TSource> items, Func<TSource, decimal> selector) 
    { 
        IEnumerable<decimal> selectedValues = items.Select(selector).Cast<decimal>(); 
        return GetMedian(selectedValues.ToArray()); 
    } 
 
    // Invoked in the following situations: 
    // - when the SourceFiled of the AggregateFunction is not set 
    // - when the corresponding data type is not supported (non numeric type) 
    // - when no method overload for the corresponding Type is defined in this class 
    public static object GetMedian<TSource, TProperty>(IEnumerable<TSource> items, Func<TSource, TProperty> selector) 
    { 
        throw new ArgumentException(string.Format("Please set the SourceField property of the {0} object. The property Type should be numeric (int, double, etc.)", nameof(MedianFunction))); 
        // or implement custom logic that fetches whatever values you need 
    } 
 
    private static double GetMedian(int[] items) 
    { 
        Array.Sort(items); 
        int mid = items.Length / 2; 
        if (items.Length % 2 != 0) 
        { 
            return items[mid]; 
        }            
        return items.Length == 2 ? (items[0] + items[1]) / 2d : (items[mid - 1] + items[mid]) / 2d; 
    } 
 
    private static double GetMedian(double[] items) 
    { 
        Array.Sort(items); 
        int mid = items.Length / 2; 
        if (items.Length % 2 != 0) 
        { 
            return items[mid]; 
        } 
        return items.Length == 2 ? (items[0] + items[1]) / 2d : (items[mid - 1] + items[mid]) / 2d; 
    }    
 
    private static decimal GetMedian(decimal[] items) 
    { 
        Array.Sort(items); 
        int mid = items.Length / 2; 
        if (items.Length % 2 != 0) 
        { 
            return items[mid]; 
        } 
        return items.Length == 2 ? (items[0] + items[1]) / 2m : (items[mid - 1] + items[mid]) / 2m; 
    } 
} 

Implementing a custom function that calculates the median of the items

• C#

public class MedianFunction : EnumerableSelectorAggregateFunction 
{ 
    protected override string AggregateMethodName 
    { 
        get { return nameof(MedianAggregates.GetMedian); } 
    } 
 
    protected override Type ExtensionMethodsType 
    { 
        get 
        { 
            return typeof(MedianAggregates); 
        } 
    } 
} 

Adding the function in the AggregateFunctions collection of RadGridView's columns

• XAML

<telerik:GridViewDataColumn DataMemberBinding="{Binding Number}"> 
    <telerik:GridViewDataColumn.AggregateFunctions> 
        <local:MedianFunction SourceField="Number"/> 
    </telerik:GridViewDataColumn.AggregateFunctions> 
</telerik:GridViewDataColumn> 

Using Generic Aggregate Function

To implement a generic aggregate function, you can use the AggregateFunction<TElement, TResult> class and set its AggregationExpression property.

Defining custom aggregate function

• C#

var customSumFunction  = new AggregateFunction<Club, int>()  
{  
    AggregationExpression = clubs => clubs.Select(x => x.StadiumCapacity).Sum()  
};  
this.gridView.Columns[0].AggregateFunctions.Add(customSumFunction);