I just created a DotNetDataSource for boolean values (e.g., "true" and "false"). I created a radio button picker with that data source, which worked fine.

However, I then wanted to default to "true" whenever a content editor comes across a new property of that data. I couldn't find a way to do that. It seems that the radio buttons are unset by default and there is no way (via code or configuration) to default them.

Am I missing something? Is there a way to set a default option?

I wouldn't think it should matter, but here's my dot net data source for your reference:

public class ConfigurableBool : IDotNetDataSource
{
    [DotNetDataSource(
        Title = "Default Value",
        Description = @"Enter the value that will be the default (i.e., ""True"" or ""False"").")]
    public string DefaultValue { get; set; }

    [DotNetDataSource(
        Title = "True Label",
        Description = @"The label to use for the ""True"" item.")]
    public string TrueLabel { get; set; }

    [DotNetDataSource(
        Title = "False Label",
        Description = @"The label to use for the ""False"" item.")]
    public string FalseLabel { get; set; }

    public IEnumerable<StringPair> GetEditorDataItems(int contextId)
    {
        var trueLabel = string.IsNullOrWhiteSpace(TrueLabel) ? "True" : TrueLabel;
        var falseLabel = string.IsNullOrWhiteSpace(FalseLabel) ? "False" : FalseLabel;
        var defaultValue = DefaultValue ?? string.Empty;

        var items = new StringPair[]
        {
            new StringPair(true.ToString(), trueLabel),
            new StringPair(false.ToString(), falseLabel)
        };
        return items
            .Select((Item, Index) => new
            {
                Item,
                Index
            })
            .OrderBy(x => x.Item.Key.InvariantEquals(DefaultValue) ? 0 : 1)
            .ThenBy(x => x.Index)
            .Select(x => x.Item).ToArray();
    }
}