Add new ConfigurationElement in ConfigurationElementCollection throws Read-only exception

Question

I have a .config file, where i need to add new ConfigurationElement under the ConfigurationElementCollection, I have followed the documentation of microsoft and implemented everything https://learn.microsoft.com/en-us/dotnet/api/system.configuration.configurationelementcollection?view=windowsdesktop-7.0.

But BaseAdd override method always throws the read-only exception.

This is my config file data, i am trying to add new element under testelements attribute.

 <testElements>

    <testElements>
    <clear/>
    <add name="test"
    model = "model"/>

This is ConfigurationElement Class:

public class TestElements: ConfigurationElement
    {
        public TestElements()
        {

        }
        public TestElements(string elementName)
        {
            Name = elementName;
        }

        [ConfigurationProperty("name",
            IsRequired = true, IsKey = true)]
        public string Name
        {
            get
            {
                return ((string)this["name"]);
            }
            set
            {
                this["name"] = value;
            }
        }

        [ConfigurationProperty("model", IsRequired = true, IsKey = true)]
        public string ModelName
        {
            get
            {
                return (string)this["model"];
            }
            set
            {
                this["model"] = value;
            }
        }

    }

My configurationelementCollection class:

public class TestElementsCollection : ConfigurationElementCollection
    {
        public TestElementsCollection()
        {
           
        }

        public override
    ConfigurationElementCollectionType CollectionType
        {
            get
            {
                return

                    ConfigurationElementCollectionType.AddRemoveClearMap;
            }
        }

        protected override
            ConfigurationElement CreateNewElement()
        {
            return new ServiceRouteDefaults();
        }


        protected override
            ConfigurationElement CreateNewElement(
            string elementName)
        {
            return new TestElements(elementName);
        }

        public new string AddElementName
        {
            get
            { return base.AddElementName; }

            set
            { base.AddElementName = value; }

        }

        public new string ClearElementName
        {
            get
            { return base.ClearElementName; }

            set
            { base.ClearElementName = value; }

        }

        public new string RemoveElementName
        {
            get
            { return base.RemoveElementName; }
        }

        public void Add(TestElements url)
        {
            BaseAdd(url);
            // Add custom code here.
        }

        protected override void
            BaseAdd(ConfigurationElement element)
        {
            BaseAdd(element, false);
            // Add custom code here.
        }
        
        public void Remove(TestElements url)
        {
            string key = GetElementKey(url).ToString();
            if (BaseIndexOf(url) >= 0)
                BaseRemove(key.ToLower());
        }

        public void RemoveAt(int index)
        {
            BaseRemoveAt(index);
        }

        public void Remove(string name)
        {
            BaseRemove(name);
        }

        public void Clear()
        {
            BaseClear();
            // Add custom code here.
        }

When trying to add using calling Add method -> Add(new TestElements() {Name = "check", ModelName ="checkname"}) -> this throws Configuration is readonly, looks BaseAdd is failing. Did I do something wrong?