Evaluate custom model attribute in more generic way

Question

I have a data model like below. One of its properties has been decorated with custom attribut - "Mergable":

public class Example
{
    [Browsable(false)]
    [Mergable(false)]
    public int ExampleId { get; set; }
}

Here's a definition of "Mergable":

[AttributeUsage(AttributeTargets.Property)]
public class MergableAttribute : Attribute
{
    public MergableAttribute()
    {
        Mergable = false;
    }

    public MergableAttribute(bool value)
    {
        Mergable = value;
    }

    public bool Mergable { get; set; } = false;
}

I use AttributeEvaluator class to evaluate the attribute:

public class AttributeEvaluator
{
    public bool Evaluate(object instance, string propertyName)
    {
        AttributeCollection attributes = TypeDescriptor.GetProperties(instance)[propertyName].Attributes;
        foreach (var a in attributes)
        {
            if (a.GetType() == typeof(MergableAttribute))
            {
                return ((MergableAttribute)a).Mergable;
            }
        }
        return false;
    }
}

I can get the value of the attribute from forms like below:

private void btnAdd_Click(object sender, EventArgs e)
    {

        bool x = new AttributeEvaluator().Evaluate(new Example(), "ExampleId");
    }

I'm wondering how I can make it more generic. Perfect situation would be to have single class to evaluate all custom attributes. I can pass the attribute type to Evaluate method, but can't cast it to appropriate type to get "Mergable" value.

public class AttributeEvaluator
{
    public bool Evaluate(object instance, string propertyName, Type attributeType)
    {
        AttributeCollection attributes = TypeDescriptor.GetProperties(instance)[propertyName].Attributes;
        foreach (var a in attributes)
        {
            if (a.GetType() == attributeType)
            {
                return ((MergableAttribute)a).Mergable; //<-- how to cast a to appopriate type?
            }
        }
        return false;
    }
}

Is there any way I can achieve that or it's just wild-goose chase?