Shader变种收集PassType的问题

ShaderVariantCollection shaderVariantCollection = new ShaderVariantCollection();
shaderVariantCollection.Add(new ShaderVariantCollection.ShaderVariant(shader, /*passtype*/, keyword));
AssetDatabase.CreateAsset(shaderVariantCollection, collectionPath);

SubShader
{
    Pass
    {
        #pragma multi_feature _ A
    }

    Pass
    {

        Tags{"LightMode" = "ShadowCaster"}
        #pragma multi_feature _ B
    }
}

for (int i = 0; i < shader.passCount; i++)
{
    string shaderTag = shader.FindPassTagValue(i, new ShaderTagId("LightMode")).name;
    PassIdentifier identifier = new PassIdentifier();
    Type myType = typeof(PassIdentifier);

    FieldInfo myFieldInfo = myType.GetField("m_PassIndex", BindingFlags.NonPublic | BindingFlags.Instance);
    object asObject = identifier;
    myFieldInfo.SetValue(asObject, (uint)i);
    identifier = (PassIdentifier)asObject;

    Debug.Log(shaderTag); //LightMode 名称 


    List<string> keyword = new List<string>();
    foreach (var item in ShaderUtil.GetPassKeywords(shader, identifier))
    {
        if(item.type== ShaderKeywordType.UserDefined)
        {
            keyword.Add(item.name);
        }
    }
    //输出LightMode定义的宏
    Debug.Log(string.Join(",", keyword.ToArray()));
}