somewhere.cs

var engine = new EngineFactory().Create(new StringContentRazorProject(
            getContentFunc: key =>
            {
                var template = _templateRepository.GetByIdAsync(Guid.Parse(key)).Result;
                if (template == null)
                    throw new Exception("Template is null");

                MemoryStream stream = new MemoryStream();
                StreamWriter writer = new StreamWriter(stream);
                writer.Write(template.BodyContent);
                writer.Flush();
                stream.Seek(0, SeekOrigin.Begin);
                return stream;
            },
            existsCheckFunc: key =>
            {
                var template = _templateRepository.GetByIdAsync(Guid.Parse(key)).Result;
                return template != null;
            }));

StringContentRazorProject.cs

public class StringContentRazorProject : RazorLightProject
{
    public StringContentRazorProject(Func<string, Stream> getContentFunc, Func<string, bool> existsCheckFunc)
    {
        this.GetContentFunc = getContentFunc;
        this.ExistsCheckFunc = existsCheckFunc;
    }

    public Func<string, Stream> GetContentFunc { get; }
    public Func<string, bool> ExistsCheckFunc { get; }

    public override Task<RazorLightProjectItem> GetItemAsync(string templateKey)
    {
        var item = new StringContentRazorProjectItem(templateKey, getContentFunc: GetContentFunc, existsCheckFunc: ExistsCheckFunc);

        return Task.FromResult((RazorLightProjectItem)item);
    }

    public override Task<IEnumerable<RazorLightProjectItem>> GetImportsAsync(string templateKey)
    {
        return Task.FromResult(Enumerable.Empty<RazorLightProjectItem>());
    }
}

StringContentRazorProjectItem.cs

public class StringContentRazorProjectItem : RazorLightProjectItem
{
    public StringContentRazorProjectItem(string key, Func<string, Stream> getContentFunc, Func<string, bool> existsCheckFunc)
    {
        Key = key;
        this.GetContentFunc = getContentFunc;
        this.ExistsCheckFunc = existsCheckFunc;
    }

    public override string Key { get; set; }
    public Func<string, Stream> GetContentFunc { get; }
    public Func<string, bool> ExistsCheckFunc { get; }

    public override bool Exists
    {
        get
        {
            return ExistsCheckFunc(Key);
        }
    }

    public override Stream Read()
    {
        return GetContentFunc(Key);
    }
}