設置EntityFramework中decimal類型數據精度

EF中默認的decimal數據精度為兩位數，當我們數據庫設置的精度大於2時，EF將只會保留到2為精度。

e.g.  2.1999將會被保存為2.20

網上找到常見的方法為重寫DbContext的OnModelCreating方法：

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{            
　　modelBuilder.Entity<Product>().Property(x => x.Price).HasPrecision(18, 4);
}

但如果數據表多或者Decimal類型字段多的話，用OnModelCreating的方法將會變得相當冗余，而且不便管理。

我推薦使用Attribute屬性標簽進行設置，在Entity Model class中decimal的字段上方直接添加自定義拓展的屬性標簽即可。

e.g.

其中 [DecimalPrecision(18, 4)]即是我們自定義的精度Attribute

具體實現代碼如下：

    /// <summary>
    /// <para>自定義Decimal類型的精度屬性</para>
    /// </summary>
    [AttributeUsage(AttributeTargets.Property, Inherited = false, AllowMultiple = false)]
    public sealed class DecimalPrecisionAttribute : Attribute
    {

        #region Field
        private byte _precision = 18;
        public byte _scale = 4;
        #endregion

        #region Construct
        /// <summary>
        /// <para>自定義Decimal類型的精確度屬性</para>
        /// </summary>
        /// <param name="precision">precision
        /// <para>精度（默認18）</para></param>
        /// <param name="scale">scale
        /// <para>小數位數（默認4）</para></param>
        public DecimalPrecisionAttribute(byte precision = 18, byte scale = 4)
        {
            Precision = precision;
            Scale = scale;
        } 
        #endregion
        
        #region Property
        /// <summary>
        /// 精確度（默認18）
        /// </summary>
        public byte Precision
        {
            get { return this._precision; }
            set { this._precision = value; }
        }

        /// <summary>
        /// 保留位數（默認4）
        /// </summary>
        public byte Scale
        {
            get { return this._scale; }
            set { this._scale = value; }
        } 
        #endregion
    }

    /// <summary>
    /// 用於modelBuilder全局設置自定義精度屬性
    /// </summary>
    public class DecimalPrecisionAttributeConvention
        : PrimitivePropertyAttributeConfigurationConvention<DecimalPrecisionAttribute>
    {
        public override void Apply(ConventionPrimitivePropertyConfiguration configuration, DecimalPrecisionAttribute attribute)
        {
            if (attribute.Precision < 1 || attribute.Precision > 38)
            {
                throw new InvalidOperationException("Precision must be between 1 and 38.");
            }
            if (attribute.Scale > attribute.Precision)
            {
                throw new InvalidOperationException("Scale must be between 0 and the Precision value.");
            }
            configuration.HasPrecision(attribute.Precision, attribute.Scale);
        }
    }

再在DbContext重寫OnModelCreating，添加自定義的DecimalPrecisionAttributeConvention即可以方便地任意添加需要精度控制的字段。

    public class Project_DbContext : DbContext
    {
        public Project_DbContext() : base("DefaultConnection") { }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Conventions.Add(new DecimalPrecisionAttributeConvention());
            base.OnModelCreating(modelBuilder);
        }

    }