public interface ICustomAggregateFunction :
    ICustomFunctionOperator

Public Interface ICustomAggregateFunction
    Inherits ICustomFunctionOperator

using DevExpress.Data.Filtering;
using DevExpress.DataAccess.ExpressionEditor;
using DevExpress.DataProcessing.Criteria;
using System;
using System.Collections.Generic;

namespace Dashboard_StringConcatAggregate {
    class StringConcatFunction : ICustomAggregateFunction, ICustomFunctionOperatorBrowsable, 
        ICustomFunctionCategory {
        public string Name => "StringConcat";

        public int MinOperandCount => 1;

        public int MaxOperandCount => 1;

        public string Description => @"Takes strings, aggregates by input value, 
            and displays them separated by commas.";

        public FunctionCategory Category => DevExpress.Data.Filtering.FunctionCategory.Text;

        public string FunctionCategory => "Aggregate";

        public object Evaluate(params object[] operands) {
            throw new NotImplementedException();
        }

        public Type GetAggregationContextType(Type inputType) { 
            return typeof(StringConcatState);   
        }

        public bool IsValidOperandCount(int count) {
            return count <= MaxOperandCount && count >= MinOperandCount;
        }

        public bool IsValidOperandType(int operandIndex, int operandCount, Type type) {
            return IsValidOperandCount(operandCount) && operandIndex == 0 && type == typeof(string);
        }

        public Type ResultType(params Type[] operands) {
            return typeof(string);
        }
    }

    class StringConcatState : ICustomAggregateFunctionContext<string, string> {
        List<string> enumeration = new List<string>();
        ISet<string> names = new HashSet<string>();
        public string GetResult() {
            return String.Join(", ", enumeration.ToArray());
        }

        public void Process(string value) {
            if (names.Add(value)) {
                enumeration.Add(value);
            }
        }
    }
}

Imports DevExpress.Data.Filtering
Imports DevExpress.DataAccess.ExpressionEditor
Imports DevExpress.DataProcessing.Criteria

Namespace Dashboard_StringConcatAggregate
    Friend Class StringConcatFunction
        Implements ICustomAggregateFunction, ICustomFunctionOperatorBrowsable, 
        ICustomFunctionCategory
        Public ReadOnly Property Name() As String Implements DevExpress.Data.Filtering.ICustomFunctionOperator.Name
            Get
                Return "StringConcat"
            End Get
        End Property
        Public ReadOnly Property MinOperandCount() As Integer Implements ICustomFunctionOperatorBrowsable.MinOperandCount
            Get
                Return 1
            End Get
        End Property
        Public ReadOnly Property MaxOperandCount() As Integer Implements ICustomFunctionOperatorBrowsable.MaxOperandCount
            Get
                Return 1
            End Get
        End Property
        Public ReadOnly Property Description() As String Implements ICustomFunctionOperatorBrowsable.Description
            Get
                Return "Takes strings, aggregates by input value, and displays them separated by commas."
            End Get
        End Property
        Public ReadOnly Property Category() As FunctionCategory Implements ICustomFunctionOperatorBrowsable.Category
            Get
                Return DevExpress.Data.Filtering.FunctionCategory.Text
            End Get
        End Property
        Private ReadOnly Property ICustomFunctionCategory_FunctionCategory As String Implements ICustomFunctionCategory.FunctionCategory
            Get
                Return "Aggregate"
            End Get
        End Property
        Public Function Evaluate(ParamArray ByVal operands() As Object) As Object Implements DevExpress.Data.Filtering.ICustomFunctionOperator.Evaluate
            Throw New NotImplementedException()
        End Function
        Public Function IsValidOperandCount(ByVal count As Integer) As Boolean Implements ICustomFunctionOperatorBrowsable.IsValidOperandCount
            Return count <= MaxOperandCount AndAlso count >= MinOperandCount
        End Function
        Public Function IsValidOperandType(ByVal operandIndex As Integer, ByVal operandCount As Integer, ByVal type As Type) As Boolean Implements ICustomFunctionOperatorBrowsable.IsValidOperandType
            Return IsValidOperandCount(operandCount) AndAlso operandIndex = 0 AndAlso type Is GetType(String)
        End Function
        Public Function ResultType(ParamArray ByVal operands() As Type) As Type Implements DevExpress.Data.Filtering.ICustomFunctionOperator.ResultType
            Return GetType(String)
        End Function
        Private Function GetAggregationContextType(inputType As Type) As Type Implements ICustomAggregateFunction.GetAggregationContextType
            Return GetType(StringConcatState)
        End Function
    End Class
    Friend Class StringConcatState
        Implements ICustomAggregateFunctionContext(Of String, String)
        Private enumeration As New List(Of String)()
        Private names As ISet(Of String) = New HashSet(Of String)()
        Public Function GetResult() As String Implements ICustomAggregateFunctionContext(Of String, String).GetResult
            Return String.Join(", ", enumeration.ToArray())
        End Function
        Public Sub Process(ByVal value As String) Implements ICustomAggregateFunctionContext(Of String, String).Process
            If names.Add(value) Then
                enumeration.Add(value)
            End If
        End Sub
    End Class
End Namespace